Departamento de Física

Received 12 February 2015
Received in revised form 14 April 2015 Accepted 21 April 2015
Available online 29 April 2015
Keywords:
ZnO:Ag nanoparticles
Optical properties
Chemical states
Local vibrational modes (LVMs)
1. Introduction
Metal oxide semiconductor nanostructures have attracted con- siderable research interest during the last several years in terms of new technological applications, mainly because of their unique properties observed only at nanosized dimensions. Zinc oxide (ZnO) is a large band gap semiconducting metal oxide with a high-exciton binding energy (60 meV) [1–3]. This binding energy allows the excitonic transitions to occur even at room temper- ature (RT), which could mean higher radiative recombination efficiency for spontaneous emissions as well as a lower thresh- old voltage for laser emission. ZnO has been investigated in the past decade due to its interesting optical and electrical properties [4,5]. Recently, doped ZnO nanostructures have attracted signif- icant attention for photocatalytic applications [6,7], where the effect of doping could greatly improve the luminescent properties
∗ Corresponding author. Tel.: +56 2 2978 4795/2978 4222; fax: +56 2 2699 4119. E-mail addresses: edemova@ing.uchile.cl (E. Mosquera),
rojasmichea@gmail.com (C. Rojas-Michea), mmorel@ing.uchile.cl
(M. Morel), Fgracia@ing.uchile.cl (F. Gracia), vfuenzal@ing.uchile.cl (V. Fuenzalida), rzarate@ucn.cl (R.A. Zárate).
http://dx.doi.org/10.1016/j.apsusc.2015.04.148
0169-4332/© 2015 Elsevier B.V. All rights reserved.
abstract
Zinc oxide nanoparticles with different amounts of incorporated silver (ZnO:Ag; 0.6, 3, 6, and 9 at.% Ag) have been successfully synthesized by a simple sol gel method. The effect of Ag content on the properties of ZnO nanoparticles have been studied by various characterization techniques. The results from X- ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (RS) suggest that elemental silver is present as a second phase. The UV–visible absorption and photoluminescence (PL) properties of the samples were also studied. PL data at room temperature reveals a strong blue emission. In addition, Raman spectroscopy results indicate a very strong A1(LO) mode resulting from oxygen vacancies and zinc interstitials. A new local vibrational mode (LVM) at 480 cm−1 induced by silver can also be observed in the Raman spectra, suggesting silver incorporation into the ZnO lattice compensating the Zn vacancies, which is consistent with the XRD results.

Simulations of the shell dynamics and normal mode analysis (NMA) are carried out to study Rydberg photoexcitation of NO in Xe and Kr solids. In the case of the NO doped Kr system we have completed a previous study on shell dynamics by focusing only on the NMA, however, in the case of the NO doped Xe system we have carried out both studies: shell dynamics and NMA. For this purpose, we have used fitted Lennard-Jones potential parameters for NO(A 2 S C )-Kr and NO(A 2 S C )-Xe interactions since they have not been reported in literature. These parameters were fitted taking into account the match of simulation results to the available spectroscopic data. The dynamics of the Xe-NO matrix yielded a great dispersion in the trajectories and a slower medium response with respect to Kr-NO matrix. The NMA have allowed us to explain the immediate shell dynamics after photoexcitation, showing the validity of harmonic approximation of potentials. The results are shown comparatively with the rest of the studied matrices (Ar-NO and Ne-NO). q

This work is devoted to the thermodynamics of gravitational clustering, a collective phenomenon with a great relevance in the N-body cosmological problem. We study a classical self-gravitating gas of identical non-relativistic particles defined on the sphere S 3 ⊂ R 4 by considering gravitational interaction that corresponds to this geometric space. The analysis is performed within microcanonical description of an isolated Hamiltonian system by combining continuum approximation and steepest descend method. According to numerical solution of resulting equations, the gravitational clustering can be associated with two microcanonical phase transitions. A first phase transition with a continuous character is associated with breakdown of SO(4) symmetry of this model. The second one is the gravitational collapse, whose continuous or discontinuous character crucially depends on the regularization of short-range divergence of gravitation potential. We also derive the thermodynamic limit of this model system, the astro-physical counterpart of Gibbs-Duhem relation, the order parameters that characterize its phase transitions and the equation of state. Other interesting behavior is the existence of states with negative heat capacities, which appear when the effects of gravitation turn dominant for energies sufficiently low. Finally, we comment the relevance of some of these results in the study of as-trophysical and cosmological situations. Special interest deserves the gravitational modification of the equation of state due to the local inhomogeneities of matter distribution. Although this feature is systematically neglected in studies about Universe expansion, the same one is able to mimic an effect that is attributed to the dark energy: a negative pressure.

Usually one can find three subjects in the first year of the syllabus of any technical engineering career, namely, calculus, general physics and programming. Being physics a matter lying on the grounds of technical engineering it becomes naturally appropriate to introduce the use of calculus and programming as useful tools in the context of a physics problem. This can be accomplished by moving some Practical Classes of Physics (problem solving) into the computer pool and by reformulating the physics problems in order to make them more appropriate for this kind of approach. In this environment, students put together, for instance, programming tools and numerical methods, along with the physical laws in order to address more realistic models, different from those which can usually be treated on the blackboard. This kind of computational physics problems increases the motivation of the engineering students by embedding them into sceneries whose models are closer to those real problems they will be facing later in their professional and scientific life. This is particularly relevant for the first year of the engineering careers when the development of this kind of professional skills is usually skipped. In the present work we will illustrate these ideas by means of the known problem of "The motion of a body subject to air drag force". The basic ideas of this work have been experienced in the physics course of first year undergraduate students of telecommunication and electronics engineering of Pinar del Río University, Cuba in 2010.