Theoretical study of band alignment in nano-porous ZnO interacting with substituted Phthalocyanines

The aim of this work is the theoretical study of the band alignment between the two components of a hybrid organic-inorganic solar-cell. The working organic molecules are metal tetra-sulphonated phthalocyanines (M-Pc) and the inorganic material is nano-porous ZnO growth in the 001 direction. The theoretical calculations are being made using the density functional theory (DFT) using a GGA functional with the SIESTA code, which projects electron wave functions and density onto a real space grid and uses as basis set a linear combination of numerical, finite-range localized atomic orbitals. We also used the DFT+U method included in the code that allows a semi-empirical inclusion of electronic correlations in the description of electronic spectra for systems such as zinc oxide.

UV laser-induced high resolution cleaving of Si wafers for micro-nano devices and polymeric waveguide characterization

In this work we propose a method for cleaving silicon-based photonic chips by using a laser based micromachining system, consisting of a ND:YVO4laser emitting at 355 nm in nanosecond pulse regime and a micropositioning system. The laser makes grooved marks placed at the desired locations and directions where cleaves have to be initiated, and after several processing steps, a crack appears and propagate along the crystallographic planes of the silicon wafer. This allows cleavage of the chips automatically and with high positioning accuracy, and provides polished vertical facets with better quality than the obtained with other cleaving process, which eases the optical characterization of photonic devices. This method has been found to be particularly useful when cleaving small-sized chips, where manual cleaving is hard to perform; and also for polymeric waveguides, whose facets get damaged or even destroyed with polishing or manual cleaving processing. Influence of length of the grooved line and speed of processing is studied for a variety of silicon chips. An application for cleaving and characterizing sol–gel waveguides is presented. The total amount of light coupled is higher than when using any other procedure.

Elastic (stress-strain) halo associated to ion-induced nano-tracks in lithium niobate: role of crystal anisotropy

The elastic strain/stress fields (halo) around a compressed amorphous nano-track (core) caused by a single high-energy ion impact on LiNbO3 are calculated. A method is developed to approximately account for the effects of crystal anisotropy of LiNbO3 (symmetry 3m) on the stress fields for tracks oriented along the crystal axes (X, Y or Z). It only considers the zero-order (axial) harmonic contribution to the displacement field in the perpendicular plane and uses effective Poisson moduli for each particular orientation. The anisotropy is relatively small; however, it accounts for some differential features obtained for irradiations along the crystallographic axes X, Y and Z. In particular, the irradiation-induced disorder (including halo) and the associated surface swelling appear to be higher for irradiations along the X- or Y-axis in comparison with those along the Z-axis. Other irradiation effects can be explained by the model, e.g. fracture patterns or the morphology of pores after chemical etching of tracks. Moreover, it offers interesting predictions on the effect of irradiation on lattice parameters

Recrystallization of amorphous nano-tracks and uniform layers generated by swift-ion-beam irradiation in lithium niobate.

The thermal annealing of amorphous tracks of nanometer-size diameter generated in lithium niobate (LiNbO3) by Bromine ions at 45 MeV, i.e., in the electronic stopping regime, has been investigated by RBS/C spectrometry in the temperature range from 250°C to 350°C. Relatively low fluences have been used (<1012 cm−2) to produce isolated tracks. However, the possible effect of track overlapping has been investigated by varying the fluence between 3×1011 cm−2 and 1012 cm−2. The annealing process follows a two-step kinetics. In a first stage (I) the track radius decreases linearly with the annealing time. It obeys an Arrhenius-type dependence on annealing temperature with activation energy around 1.5 eV. The second stage (II) operates after the track radius has decreased down to around 2.5 nm and shows a much lower radial velocity. The data for stage I appear consistent with a solid-phase epitaxial process that yields a constant recrystallization rate at the amorphous-crystalline boundary. HRTEM has been used to monitor the existence and the size of the annealed isolated tracks in the second stage. On the other hand, the thermal annealing of homogeneous (buried) amorphous layers has been investigated within the same temperature range, on samples irradiated with Fluorine at 20 MeV and fluences of ∼1014 cm−2. Optical techniques are very suitable for this case and have been used to monitor the recrystallization of the layers. The annealing process induces a displacement of the crystalline-amorphous boundary that is also linear with annealing time, and the recrystallization rates are consistent with those measured for tracks. The comparison of these data with those previously obtained for the heavily damaged (amorphous) layers produced by elastic nuclear collisions is summarily discussed.

Optimization of a label-free biosensor vertically characterized based on a periodic lattice of high aspect ratio SU-8 nano-pillars with a simplified 2D theoretical model

We have recently demonstrated a biosensor based on a lattice of SU8 pillars on a 1 μm SiO2/Si wafer by measuring vertically reflectivity as a function of wavelength. The biodetection has been proven with the combination of Bovine Serum Albumin (BSA) protein and its antibody (antiBSA). A BSA layer is attached to the pillars; the biorecognition of antiBSA involves a shift in the reflectivity curve, related with the concentration of antiBSA. A detection limit in the order of 2 ng/ml is achieved for a rhombic lattice of pillars with a lattice parameter (a) of 800 nm, a height (h) of 420 nm and a diameter(d) of 200 nm. These results correlate with calculations using 3D-finite difference time domain method. A 2D simplified model is proposed, consisting of a multilayer model where the pillars are turned into a 420 nm layer with an effective refractive index obtained by using Beam Propagation Method (BPM) algorithm. Results provided by this model are in good correlation with experimental data, reaching a reduction in time from one day to 15 minutes, giving a fast but accurate tool to optimize the design and maximizing sensitivity, and allows analyzing the influence of different variables (diameter, height and lattice parameter). Sensitivity is obtained for a variety of configurations, reaching a limit of detection under 1 ng/ml. Optimum design is not only chosen because of its sensitivity but also its feasibility, both from fabrication (limited by aspect ratio and proximity of the pillars) and fluidic point of view. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High frequency high-order Rayleigh modes in ZnO/GaAs

Strong high-order Rayleigh or Sezawa modes, in addition to the fundamental Rayleigh mode, have been observed in ZnO/GaAs(001) systems along the [110] propagation direction of GaAs. The dispersion of the different acoustic waves has been calculated and compared to the experimental data. The bandwidth and impedance matching characteristics of the multimode SAW delay lines operating at high frequencies (2.5-3.5 GHz regime) have been investigated.

Processing and adsorption control in ZnO single nanowire photodetectors

ZnO single nanowire photodetectors have been measured in different ambient conditions in order to understand and control adsorption processes on the surface. A decrease in the conductivity has been observed as a function of time when the nanowires are exposed to air, due to adsorbed O2/H2O species at the nanowire surface. In order to have a device with stable characteristics in time, thermal desorption has been used to recover the original conductivity followed by PMMA coating of the exposed nanowire surface.

Influencia de la adición de nano sílice en el hormigón autocompactante

En los últimos años es notable la proliferación de trabajos y estudios que tratan sobre las características del hormigón autocompactante. De ellos, la durabilidad es el aspecto menos tratado, siendo especialmente escasos los que se centran en un problema particular de esta durabilidad, como es la penetración de cloruros, un aspecto básico para todos los elementos estructurales sometidos a un ambiente marino. Esta será la línea básica del presente trabajo, que vendrá acompañada de otra serie de ensayo que permitan ratificar los resultados obtenidos. Debido a lo anteriormente expuesto, el objetivo general de esta investigación es estudiar la influencia de la adición de nano-sílice en aspectos tanto microestructurales como durables en hormigones autocompactantes. Dado que el objetivo general planteado es muy ambicioso y requiere tiempo y multitud de ensayos combinando numerosas variables, este trabajo de investigación se centra en los siguientes objetivos particulares dentro de la línea general de la investigación: Evaluar los cambios que se producen en las propiedades en estado fresco de los distintos hormigones ensayados; Evaluar los cambios que se producen en las propiedades mecánicas de los hormigones estudiados; Determinar los cambios de la matriz porosa de los distintos hormigones ensayados y determinar los cambios en los componentes hidratados de la matiz de cemento. Para cumplir con este objetivo, se ha procedido a comparar el comportamiento de cuatro tipos de hormigón con el mismo cemento: Un hormigón convencional sin adición, un hormigón autocompactante sin adición, un hormigón autocompactante con 2,5 % de adición de nano sílice y un hormigón autocompactante con 5 % de adición de nano sílice. Las etapas seguidas en este trabajo son las siguientes: Revisión bibliográfica relativa a los hormigones autocompactantes, y a la adición de nano-sílice.; Estudio y elección de las dosificaciones para los hormigones objeto de estudio: hormigón convencional, un hormigón autocompactante sin adiciones y hormigones autocompactantes con adición de nano-sílice; Evaluación de los hormigones, convencional y autocompactantes, en estado fresco en base a la normativa vigente y a las exigencias de la Instrucción del Hormigón Estructural (EHE-08); Evaluación de las propiedades mecánicas de los hormigones en estado endurecido mediante ensayo de resistencia a compresión; Caracterización microestructural de los hormigones mediante ensayos de porosimetría por intrusión de mercurio (PIM) y termoanálisis (TG-ATD); Evaluación del comportamiento de los hormigones frente a ensayos específicos enfocados a su durabilidad, como son los de resistividad eléctrica y de penetración de cloruros y estudio comparativo de los resultados obtenidos y establecimiento de relaciones entre la dosificación y el comportamiento de cada hormigón, de cara a poder fijar recomendaciones de uso.

E-beam nano-patterning for the ordered growth of GaN/InGaN nanorods

E-beam lithography was used to pattern a titanium mask on a GaN substrate with ordered arrays of nanoholes. This patterned mask served as a template for the subsequent ordered growth of GaN/InGaN nanorods by plasma-assisted molecular beam epitaxy. The mask patterning process was optimized for several holes configurations. The smallest holes were 30 nm in diameter with a pitch (center-to-center distance) of 100 nm only. High quality masks of several geometries were obtained that could be used to grow ordered GaN/InGaN nanorods with full selectivity (growth localized inside the nanoholes only) over areas of hundreds of microns. Although some parasitic InGaN growth occurred between the nanorods during the In incorporation, transmission electron microscopy and photoluminescence measurements demonstrated that these ordered nanorods exhibit high crystal quality and reproducible optical properties.

Selective area growth and characterization of InGaN nano-disks implemented in GaN nanocolumns with different top morphologies

This work reports on the morphology control of the selective area growth of GaN-based nanostructures on c-plane GaN templates. By decreasing the substrate temperature, the nanostructures morphology changes from pyramidal islands (no vertical m-planes), to GaN nanocolumns with top semipolar r-planes, and further to GaN nanocolumns with top polar c-planes. When growing InGaN nano-disks embedded into the GaN nanocolumns, the different morphologies mentioned lead to different optical properties, due to the semi-polar and polar nature of the r-planes and c-planes involved. These differences are assessed by photoluminescence measurements at low temperature and correlated to the specific nano-disk geometry.

Electrophoretic Deposition of Transparent ZnO Thin Films from Highly Stabilized Colloidal Suspensions

The parameters that control the stability of ZnO-nanoparticles suspensions and their deposition by electrophoretic deposition were studied, so as to organize the assembly and compaction of nanoparticles. The addition of cationic polyelectrolyte - Polyethylenimine (PEI) - with different molecular weights was investigated, in order to study their effectiveness and the influence of the molecular weight of the organic chain on suspensions dispersion. It was found that PEI with the highest molecular weight provided better dispersion conditions. Cathodic EPD was performed under previously optimized suspensions conditions and over electropolished stainless steel substrates. Experimental results showed that the EPD process in these conditions allows obtaining dense transparent ZnO thin films. Deposition times and intensities were optimized by analyzing the resulting thin films characteristics. Finally, the deposits were characterized by FE-SEM, AFM, and different spectroscopic techniques.

Estructura electrónica de fotocatalizadores compuestos ZnO-TiO2

Recientes publicaciones han mostrado propiedades fotocatalíticas interesantes en sistemas basados en TiO2 y ZnO. En unos casos hay presentes fases de ambos óxidos binarios en íntimo contacto [1] y en otros se tienen óxidos mixtos (titanatos de Zn) de distintas estequiometrías [2]; estos últimos, además, se han podido dopar con nitrógeno para obtener actividad con luz visible [3]. Las características electrónicas relevantes de estos sistemas (posición relativa de los niveles de ambas fases en el primero, estructura de bandas para los titanatos con o sin N) se conocen muy poco. Aquí se realiza un estudio teórico cuántico de estos materiales, usando para mayor exactitud funcionales híbridos (pues es sabido que la DFT estándar predice mal los bandgaps). Además se tienen en cuenta desarrollos teóricos recientes que permiten determinar ab initio, para semiconductores de gap alto, el coeficiente más adecuado de mezcla de intercambio HF [4, 5], y formular reglas para obtener con más exactitud el alineamiento de bandas que se establece a través de una interfaz [5, 6].

Nano-imprinted rear-side diffraction gratings for absorption enhancement in solar cells

As wafer-based solar cells become thinner, light-trapping textures for absorption enhancement will gain in importance. In this work, crystalline silicon wafers were textured with wavelength-scale diffraction grating surface textures by nanoimprint lithography using interference lithography as a mastering technology. This technique allows fine-tailored nanostructures to be realized on large areas with high throughput. Solar cell precursors were fabricated, with the surface textures on the rear side, for optical absorption measurements. Large absorption enhancements are observed in the wavelength range in which the silicon wafer absorbs weakly. It is shown experimentally that bi-periodic crossed gratings perform better than uni-periodic linear gratings. Optical simulations have been made of the fabricated structures, allowing the total absorption to be decomposed into useful absorption in the silicon and parasitic absorption in the rear reflector. Using the calculated silicon absorption, promising absorbed photocurrent density enhancements have been calculated for solar cells employing the nano-textures. Finally, first results are presented of a passivation layer deposition technique that planarizes the rear reflector for the purpose of reducing the parasitic absorption.

Durabilidad y propiedades mecánicas del hormigón autocompactante con adición de microsílice y nano sílice

Con la creciente preocupación por el impacto ambiental de la industria del cemento, el uso de subproductos industriales en la fabricación del cemento y del hormigón ha cobrado gran interés en nuestros días, ya que permiten utilizar estos desechos, reduciendo en muchos casos el consumo de cemento en el hormigón; disminuyendo a su vez el consumo energético y las emisiones asociadas a su producción. El humo de sílice es un subproducto de la producción de metal silicio o ferrosilício, su uso como adición en el hormigón ha demostrado ser efectivo en hormigones de alta resistencia. Las partículas de SiO2 micro y nano, reaccionan con el hidróxido de calcio y la tasa de la reacción puzolánica es proporcional al área disponible para la reacción. Al utilizar nano-partículas, se aumenta el área, mejorando seguramente el desempeño del hormigón. Por lo tanto, resulta de utilidad estudiar el efecto de estas adiciones en el hormigón. En este trabajo se han estudiado las propiedades mecánicas y la durabilidad de varias mezclas de hormigón autocompactante de alta resistencia con adición de microsílice y nanosílice. Para este propósito se ha incorporado a la mezcla de hormigón microsílice en 3, 6 y 10% y nanosílice en 3 y 6% respectivamente, en peso de cemento. En todas las mezclas se han mantenido constantes el contenido de cemento de 450 kg/m3 , la relación a/c y la cantidad de aditivo plastificante. Se han realizado ensayos en estado fresco para obtener las características reológicas de los hormigones. Se han determinado las principales propiedades mecánicas a 28 días de edad y se han realizado ensayos de durabilidad. Los resultados de los ensayos muestran una mejora significativa en las propiedades del hormigón. Desde el punto de vista microestructural se observa una estructura porosa más refinada y densa en las mezclas que contienen adiciones, lo cual puede contribuir al mejoramiento de la resistencia y la durabilidad de los hormigones. Las nanopartículas resultaron ser mejores que el microsílice en el aumento de la resistencia.

Electrical, photoelectrical and morphological properties of ZnO nanowire networks grown on SiO2 and on Si

ZnO nanofibre networks (NFNs) were grown by vapour transport method on Si-based substrates. One type of substrate was SiO2 thermally grown on Si and another consisted of a Si wafer onto which Si nanowires (NWs) had been grown having Au nanoparticles catalysts. The ZnO-NFN morphology was observed by scanning electron microscopy on samples grown at 600 °C and 720 °C substrate temperature, while an focused ion beam was used to study the ZnO NFN/Si NWs/Si and ZnO NFN/SiO2 interfaces. Photoluminescence, electrical conductance and photoconductance of ZnO-NFN was studied for the sample grown on SiO2. The photoluminescence spectra show strong peaks due to exciton recombination and lattice defects. The ZnO-NFN presents quasi-persistent photoconductivity effects and ohmic I-V characteristics which become nonlinear and hysteretic as the applied voltage is increased. The electrical conductance as a function of temperature can be described by a modified three dimensional variable hopping model with nanometer-ranged typical hopping distances.