Scielo RSS <![CDATA[Superficies y vacío]]> http://www.scielo.org.mx/rss.php?pid=1665-352120140001&lang=pt vol. 27 num. 1 lang. pt <![CDATA[SciELO Logo]]> http://www.scielo.org.mx/img/en/fbpelogp.gif http://www.scielo.org.mx <![CDATA[<b>Memristance effect of metal-insulator-metal structures using Al<sub>2</sub>O<sub>3</sub> film as active layer for</b> <b>emergent memory devices</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000100001&lng=pt&nrm=iso&tlng=pt The memristor effect (cyclic switching from a High-Resistance State to a Low-Resistance State and viceversa) and a conductivity/resistivity window (I OFF/I ON ratio) of more than 4 orders of magnitude have been obtained in Metal-Insulator-Metal (MIM) structures that use Al2O3 as oxide layer after cyclic I-V measurements. Before initial breakdown, different carrier conduction properties are found when using whether a highly reactive or a highly inert metal as the bottom electrode of the memory structure. These carrier conduction properties for the MIM structure are also affected by changing the deposition conditions of the bottom electrode (by changing the average roughness of its surface) or by using different Al2O3 thickness. Most importantly, because of the low thermal budget used for fabrication of these devices (only 300°C), they have the potential to be vertically integrated as memory cells in higher metallization levels of a CMOS-based Back-End-Of-Line (BEOL) processing. This is important since it would promote real 3-D integration of memory devices for advanced circuit manufacturing. <![CDATA[<b>Morphological characterization of zinc oxide nanowires grown hydrothermally on PET</b> <b>substrate</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000100002&lng=pt&nrm=iso&tlng=pt In this work, we report the synthesis and morphological characterization of ZnO nanowires grown on PET using a two-step process: (a) preparation of the seed layer, and (b) growth of the nanostructures. First step: solutions of zinc acetate dehydrate and 1-propanol of 10 mM concentration were spin coated on PET at 2000 rpm for 54 s using a syringe pump. Three of such cycles were carried out, between each cycle the layer was heating at 100°C in a furnace for 60 s. A seed layer was obtained after three layers of spin coating. Second step: vertical ZnO nanowires were grown by dipping the substrate in a 25 mM equimolar solution of zinc nitrate hexahydrate and hexamethylenetetramine. Afterwards, films were thermally treated with a commercial microwave oven at different power settings (350 and 700 W) for 5, 20 and 35 min. The ZnO nanowires obtained were characterized structural, optical and morphologically. Scanning electron microscopy analysis showed that the size of ZnO nanowires decreased in diameter when the heating time increased. XRD patterns show the presence of Zn(OH)2 overlapped with the ZnO phase. The ZnO nanowires bandgap energy was obtained from optical transmission spectra. <![CDATA[<b>Obtaining and characterization of ZnSe nanoparticles from aqueous colloidal dispersions</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000100003&lng=pt&nrm=iso&tlng=pt Structural, morphological and compositional characterizations of zinc selenide (ZnSe) nanoparticles (NPs) are presented. ZnSe NPs have been obtained by colloidal synthesis in aqueous solution using zinc nitrate (Zn(NO3)2) and elemental selenium (Se) as precursors, sodium borohydride (NaBH4) as reducing agent, a solution of sodium hydroxide (NaOH) and pentasodium tripolyphosphate (Na5P3O10) named Extran was used as surfactant. The pH was varied from 8 to 11 and the Zn:Se molar concentration was varied from 3:1 to 1:3. A cleaning process was applied to eliminate the by-products using hydrochloric acid (HCl). ZnSe NPs exhibit a cubic zinc-blende structure. The NPs size was estimated between 4.9 nm and 3.0 nm and depends on molar concentration. <![CDATA[<b>Effects of temperature and deposition time on the RF- sputtered CdTe films preparation</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000100004&lng=pt&nrm=iso&tlng=pt In this work, CdTe thin films were deposited by rf-sputtering at different substrate temperatures (room temperature (RT), 100, 150, 200, and 250 °C) and deposition times (30, 60, and 90 min). The applied power and vacuum pressure were maintained fixed for all depositions. A mean value of 18.5 nm/s on the deposition rate was maintained for films deposition. The surface morphology, rms-roughness, and grain size of the sputtered-films were obtained from atomic force microscopy and scanning electron microscopy images for comparison. CdTe films deposited during 60 min presented high reproducibility, because their consistent thickness value (1.2 μm) at different substrate temperatures. Higher thickness than 1.81 μm was obtained for films deposited at 250 °C and 90 min. CdTe films deposited at 150 °C present minor dispersion on thickness, while rms-roughness increased with the increase of substrate temperature but only for films deposited at 60 and 90 min. CdTe film deposited at 250 °C during 90 min was selected as appropriate for CdS/CdTe solar cells preparation. <![CDATA[<b>Plasma gradient modified scaffolds to generate a chemoattractant surface</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000100005&lng=pt&nrm=iso&tlng=pt A gradient of plasma polymerization of pyrrole was generated to model a chemoattractant surface; it was obtained by changing the polymerization time to manipulate the thickness and the deposited layer. The polymerization was generated with a power of 30W on a cylindrical vacuum reactor, the coverslip was polymerized covered by a cavity with a surface exposed of 0.7 cm every 35 min., the cells seeded were liver stellate cells (Ito); it was evaluated against a coverslip with continuous plasma polymerization of pyrrole (Ppy). The results show similar protein secretions, but cells proliferation was selective in the surface gradient regions. <![CDATA[<b>Design and simulation of a membranes-based acoustic sensors array for cochlear implant</b> <b>applications</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000100006&lng=pt&nrm=iso&tlng=pt In this work we present the design and simulation of an acoustic sensors array for detecting specific frequencies in a range, and allowing their identification from a complex audio signal in order to develop an application for cochlear implants. The sensitivity is obtained when a membrane resonates when stimulated by an audio signal which behaves as a mechanical wave. The structure of each sensor is designed with the concept of MEMS capacitive microphones, formed by a flexible membrane between two rigid backplates. The variation of the distance between the membrane and backplates generates a capacitance change that can pass to an electronic readout circuit. The structure has been designed as a mass-spring system where a set of springs, with the same elastic properties, holds the flexible membrane. <![CDATA[<b>Técnica de medición de baja corriente para un convertidor termoiónico</b>]]> http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1665-35212014000100007&lng=pt&nrm=iso&tlng=pt Se presenta una técnica analógica para medir las pequeñas corrientes que son generadas por un convertidor termoiónico cuando es operado en el régimen de baja temperatura. La operación en estado estable de un convertidor termoiónico (temperatura de emisor > 1500K), manifiesta una corriente dominada por el electrodo emisor. En régimen de baja temperatura puede dominar la corriente proveniente del colector; es decir, se obtiene una corriente inversa. Es importante estudiar esta corriente pues está asociada a las temperaturas de operación de los electrodos y a sus correspondientes funciones de trabajo. En esta condición la corriente inversa es de magnitud pequeña y puede mostrar valores cercanos a 1 µA. En este trabajo se emplea un capacitor de pequeña magnitud para medir en forma simultánea el voltaje y la corriente provenientes del convertidor termoiónico. La carga del capacitor se asocia a un amplificador operacional empleado como circuito buffer. Se hace un tratamiento matemático del voltaje transitorio del capacitor para obtener la corriente del convertidor termoiónico. Con esta técnica fue posible medir corrientes del orden de 1 µA.<hr/>An analog electrical technique is presented to measure a small current that is generated by a thermionic converter when it is operated at low temperature regime. Direct and inverse currents are generated simultaneously in a thermionic converter at steady state (emitter temperature > 1500K); although, at low temperature regime any of them may be more significant. Important information is obtained from a thermionic converter when it is operated at low temperature regime, particularly, when the inverse current is generated from the collector to the emitter (inverse current ≈ 1 µA). An operational amplifier is used as a buffer circuit to measure the voltage in a small capacitor. A mathematical treatment was made from the voltage data obtained from the charged capacitor to measure indirectly the current generated by the thermionic converter. It was possible to measure a small current like 1 µA by this technique.