Angel Barranco

Reversible Superhydrophobic to Superhydrophilic Conversion of Ag@TiO2 Composite Nanofiber Surfaces

A new type of superhydrophobic material consisting of a surface with supported Ag@TiO(2) core-shell nanofibers has been prepared at low temperature by plasma-enhanced chemical vapor deposition (PECVD). The fibers are formed by an inner nanocrystalline silver thread which is covered by a TiO(2) overlayer. Water contact angles depend on the width of the fibers and on their surface concentration, reaching a maximum wetting angle close to 180 degrees for a surface concentration of approximately 15 fibers microm(-2) and a thickness of 200 nm. When irradiated with UV light, these surfaces become superhydrophilic (i.e., 0 degrees contact angle). The decrease rate of the contact angle depends on both the crystalline state of the titania and on the size of the individual TiO(2) domains covering the fibers. To the best of our knowledge, this is one of the few examples existing in the literature where a superhydrophobic surface transforms reversibly into a superhydrophilic one as an effect of light irradiation.

Electronic state characterization of SiOx thin films prepared by evaporation

SiOx thin films with different stoichiometries from SiO1.3 to SiO1.8 have been prepared by evaporation of silicon monoxide in vacuum or under well-controlled partial pressures of oxygen (P<10−6Torr). These thin films have been characterized by x-ray photoemission and x-ray-absorption spectroscopies, this latter at the Si K and L2,3 absorption edges. It has been found that the films prepared in vacuum consists of a mixture of Si3+ and Si+ species that progressively convert into Si4+ as the partial pressure of oxygen during preparation increases. From this spectroscopic analysis, information has been gained about the energy distribution of both the full and empty states of, respectively, the valence and conduction bands of SiOx as a function of the O∕Si ratio. The characterization of these films by reflection electron energy-loss spectroscopy (REELS) has provided further evidences about their electronic structure (band gap and electronic states) as a function of the oxygen content. The determination of the ...

Chemical stability of Sin+ species in SiOx (x<2) thin films

SiOx thin films have been prepared by evaporation of silicon monoxide powder in an ultrahigh vacuum chamber. The films are characterized by x-ray photoelectron spectroscopy (XPS), synchrotron photoemission, and x-ray absorption spectroscopy at the Si K edge. XPS shows that the films prepared by evaporation in ultrahigh vacuum have a SiO1.3 stoichiometry and are formed by Si3+(∼77%) and Si+(∼23%) species. Based on extended x-ray absorption fine structure analysis, the structure of these films has been described as formed by tetrahedra of the type Si–(Si, O3) and Si–(Si3, O), in agreement with the Si 2p photoelectron spectra. No significant amount of Si2+ species [i.e., Si–(Si2, O2)] tetrahedra) or elemental silicon were detected in these films. When SiOx thin films are prepared by evaporation of silicon monoxide in O2 atmosphere, the oxygen content in the film increases with the partial pressure of this gas. Under these conditions, Si4+ species are formed in detriment of the Si+ and Si3+ oxidation states. ...

Differences in n-type doping efficiency between Al- and Ga-ZnO films

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices.

TiO2–SiO2 one-dimensional photonic crystals of controlled porosity by glancing angle physical vapour deposition

Herein we present a synthetic route to attain porous one-dimensional photonic crystals of high optical quality. The method employed, based on the alternate deposition of TiO2 and SiO2 porous layers by glancing angle physical vapour deposition, yields a highly accessible interconnected pore network throughout the entire multilayer structure. Furthermore, it allows a strict control over the average size and density of the interstitial sites, which results in the precise tuning of the refractive index of the individual layers and thus of the optical response of the ensemble. The controlled environmental response of the multilayer is confirmed by the optical monitoring of the infiltration of liquids of different refractive index.

Aligned TiO2 nanocolumnar layers prepared by PVD-GLAD for transparent dye sensitized solar cells

Transparent thin film electrodes made of vertically aligned nanocolumns of TiO2 with well-controlled oblique angles were grown by physical vapor deposition at glancing incidence (PVD-GLAD). For an electrode thickness of 500 nm, we report a 40% variation on solar cell efficiency (from 0.6% to 1.04%) when the deposition angle was modified between 60° and 85°. Transparent thicker films with higher surface area deposited at the optimal angle of 70° were grown with a zigzag morphology which confers high mechanical strength to the thin films. Using this topology, the application of an electrode thickness of 3 µm in a DSC resulted in a power conversion efficiency of 2.78% maintaining electrode transparency.

Active and optically transparent tetracationic porphyrin/TiO2 composite thin films

Fluorescent tetracationic porphyrin (TMPyP) molecules have been incorporated into optically transparent TiO(2) thin films acting as a host material. The films, with a columnar structure and open pores, were prepared by electron evaporation at glancing angles (GAPVD). The open porosity of the films has been estimated by measuring a water adsorption isotherm with a quartz crystal monitor. TMPyP molecules were infiltrated in the host thin films by their immersion into water solutions at controlled values of pH. The state of the adsorbed molecules, the infiltration efficiency, and the adsorption kinetics were assessed by analyzing the optical response of the films by UV-vis absorption and fluorescence techniques. The infiltration efficiency was directly correlated with the acidity of the medium, increasing at basic pHs as expected from simple considerations based on the concepts of the point of zero charge (PZC) developed for colloidal oxides. By a quantitative evaluation based on the analysis of the UV spectra, the infiltration process has been described by a Langmuir type adsorption isotherm and an Elovich-like kinetics. The accessibility of the infiltrated molecules in the TMPyP/TiO(2) composite films is assessed by following the changes of their optical properties when exposed to the acid vapors and their subsequent recovery with time.

Synthesis of SiO2 and SiOxCyHz thin films by microwave plasma CVD

Abstract Plasma enhanced chemical vapour deposition of SiO2 and polymeric SiOxCyHz thin films has been carried out at room temperature in a microwave electron cyclotron resonance (ECR) reactor. Si(CH3)3Cl has been used as volatile precursor of Si and pure oxygen as plasma gas. Plasma conditions were characterised by optical emission spectroscopy as a function of the relative flow rates of oxygen and precursor. The oxygen plasma was characterised by emission lines and bands due to O* and O2+ species whose relative intensity decreased as the flow rate of the precursor increased. Then, the plasma was dominated by the emission lines of H* species formed by dissociation of the precursor molecules. From the evolution of the intensity of the emission of oxygen and hydrogen lines as a function of the relative concentration of oxygen and precursor and by considering the composition and microstructure of the obtained thin films, a model is proposed for the decomposition mechanism of the precursor. According to this model, Si–Cl bond would dissociate in a first step. Then a series of reactions would follow with the activated oxygen species that, depending on the relative flow rate of oxygen, lead to the formation of SiO2 or a polymeric SiOxCyHz material. The chemical composition of the films was analysed by Rutherford backscattering spectroscopy (RBS), electron recoil detection analysis (ERDA) and X-ray photoelectron spectroscopy (XPS), while their structure and microstructure were investigated by means of transmission electron microscopy (TEM), atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FT-IR). It has been also shown that the SiOxCyHz thin films, with typical compositions such as Si:1, O:2, C:3.6, H:5.5, yield SiO2 thin films by exposure to a plasma of oxygen. These SiO2 thin films were smoother than the parent SiOxCyHz samples and the silica films prepared by PECVD under oxygen rich conditions.

Correlation lengths, porosity and water adsorption in TiO 2 thin films prepared by glancing angle deposition

This paper reports a thorough microstructural characterization of glancing angle deposited (GLAD) TiO(2) thin films. Atomic force microscopy (afm), grazing-incidence small-angle x-ray scattering (GISAXS) and water adsorption isotherms have been used to determine the evolution of porosity and the existence of some correlation distances between the nanocolumns constituting the basic elements of the films nanostructure. It is found that the deposition angle and, to a lesser extent, the film thickness are the most important parameters controlling properties of the thin film. The importance of porosity and some critical dimensions encountered in the investigated GLAD thin films is highlighted in relation to the analysis of their optical properties when utilized as antireflective coatings or as hosts and templates for the development of new composite materials.

Low temperature synthesis of dense SiO2 thin films by ion beam induced chemical vapor deposition

Abstract This paper presents a comparative study of SiO 2 thin films prepared at room temperature by ion beam induced chemical vapor deposition (IBICVD) and plasma enhanced chemical vapor deposition (PECVD) methods. The films are characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), Rutherford backscattering spectroscopy (RBS), electron recoil detection analysis (ERDA), nuclear reaction analysis (NRA), X-ray reflectometry and spectroscopic ellipsometry. While the films prepared by IBICVD are very compact and dense and have a high refractive index ( n =1.48 at λ=550 nm), those prepared by PECVD exhibit a lower refractive index value ( n =1.45 at λ=550 nm), lower density and have a higher surface roughness. The different microstructure and properties of the two sets of films are discussed in relation to the ballistic effects that occur by the action of the highly energetic ion beams (e.g. 400 eV) impinging on the surface of the films prepared by IBICVD.