Reactivity and oxidation of FexNi13-x nanoalloys

First-principles calculations were conducted for charged iron-nickel nanoalloys to study their structural and electronic properties. These can help in determining their reactivity and stability, to asses if they are good candidates as catalysts for some chemical reactions, and to determine optimal geometries and chemical compositions in this context. We calculated global reactivity indicators (such as electronegativity) that allow us to foresee the best chemical compositions for oxidation, and local reactivity indicators (Fukui functions) that predict which sites of the clusters are, on average, the most reactive ones. Oxygen absorption calculations for the most and least electronegative clusters have also been made to _nd out which sites are the optimal for oxygen molecules adsorption.Departamento de Física Teórica, Atómica y ÓpticaMáster en Físic

Estudio de la conmutación resistiva a partir de la caracterización eléctrica de estructuras MIM

En los últimos años, la búsqueda de sustitutos para la actual tecnología CMOS en la que se basan la mayoría de memorias de estado sólido, como las flash, se ha incrementado. Ello ha causado que fenómenos como la conmutación resistiva (resistive switching o RS) se estudien cada vez más, ya que pueden ser la base de una nueva familia de dispositivos de almacenamiento emergentes, entre los cuales están las memorias RAM resistivas, denominadas ReRAMs o RRAMs. Esta tesis se centra principalmente en la caracterización de estructuras metal-aislante-metal (MIM) que presentan conmutación resistiva, cuyo aislante está constituido por un óxido de metal de transición. El trabajo que aquí se presenta estudia en detalle la conmutación resistiva en estructuras metal-aislante-metal, con especial interés (aunque no únicamente) en el estudio de dispositivos cuyo dieléctrico es óxido de hafnio, con el objetivo de caracterizar eléctricamente el materialDepartamento de Electricidad y ElectrónicaDoctorado en Físic

Estructura electrónica y propiedades magnéticas de nanoaleaciones de FeNi

En el trabajo se presentan cálculos de primeros principios de clusters neutros de nanoaleaciones de Hierro-Níquel compuestos por 13 átomos, realizados mediante la teoría funcional de la densidad (DFT), en la aproximación de gradientes generalizados (GGA), para posteriormente hacer un análisis de su estructura electrónica y propiedades magnéticas.Grado en Físic

Genomic lineages of Rhizobium etli revealed by the extent of nucleotide polymorphisms and low recombination

<p>Abstract</p> <p>Background</p> <p>Most of the DNA variations found in bacterial species are in the form of single nucleotide polymorphisms (SNPs), but there is some debate regarding how much of this variation comes from mutation versus recombination. The nitrogen-fixing symbiotic bacteria <it>Rhizobium etli </it>is highly variable in both genomic structure and gene content. However, no previous report has provided a detailed genomic analysis of this variation at nucleotide level or the role of recombination in generating diversity in this bacterium. Here, we compared draft genomic sequences versus complete genomic sequences to obtain reliable measures of genetic diversity and then estimated the role of recombination in the generation of genomic diversity among <it>Rhizobium etli</it>.</p> <p>Results</p> <p>We identified high levels of DNA polymorphism in <it>R. etli</it>, and found that there was an average divergence of 4% to 6% among the tested strain pairs. DNA recombination events were estimated to affect 3% to 10% of the genomic sample analyzed. In most instances, the nucleotide diversity (π) was greater in DNA segments with recombinant events than in non-recombinant segments. However, this degree of recombination was not sufficiently large to disrupt the congruence of the phylogenetic trees, and further evaluation of recombination in strains quartets indicated that the recombination levels in this species are proportionally low.</p> <p>Conclusion</p> <p>Our data suggest that <it>R. etli </it>is a species composed of separated lineages with low homologous recombination among the strains. Horizontal gene transfer, particularly via the symbiotic plasmid characteristic of this species, seems to play an important role in diversity but the lineages maintain their evolutionary cohesiveness.</p

2020 IEEE Latin America Electron Devices Conference (LAEDC)

Producción CientíficaThree topologies of TiN/Ti/HfO 2 /W resistive switching memories (RRAM) are proposed in this work: crossbar, isolated and isolated-crossbar configurations. All configurations use the same sequence of technological processes. The different topologies are obtained by customizing the layouts corresponding to the bottom electrode (W), and the silicon oxide layer that is deposited on the bottom electrode. A comparative study of the resistive switching mechanisms in the three configurations has been carried out. DC current-voltage cycles and small signal conductance memory maps of single RRAM show relevant differences among the three topologies. Complex structures containing various devices (series, anti-series, parallel, antiparallel) have also been fabricated. Switching loops and memory maps obtained for these complex structures demonstrate that they are fully operative, validating the technological route to manufacture complete RRAM memory chips.Ministerio de Economía, Industria y Competitividad - Fondo Europeo de Desarrollo Regional (grants TEC2017-84321- C4-1-R and TEC2017-84321-C4-2-R

Atomic layer deposited nanolaminates of zirconium oxide and manganese oxide from manganese(III)acetylacetonate and ozone

Producción CientíficaAtomic layer deposition method was used to grow thin films consisting of ZrO2 and MnOx layers. All depositions were carried out at 300 ºC. Some deposition characteristics of the manganese(III)acetylacetonate and ozone process were investigated, such as crystallinity and the dependence of growth rate on the deposition temperature. All films were partly crystalline in their as-deposited state. Zirconium oxide contained cubic and tetragonal phases of ZrO2, while the manganese oxide was shown to consist of cubic Mn2O3 and tetragonal Mn3O4 phases. All the films exhibited nonlinear saturative magnetization with hysteresis, as well as resistive switching characteristics.Fondo Europeo de Desarrollo Regional (projects TK134 and TK141)Ministerio de Economía, Industria y Competitividad (project TEC2017-84321-C4-2-R)Estonian Research Agency (projects PRG4 and PRG753

Performance Assessment of Amorphous HfO2-Based RRAM Devices for Neuromorphic Applications

Producción CientíficaThe use of thin layers of amorphous hafnium oxide has been shown to be suitable for the manufacture of Resistive Random-Access memories (RRAM). These memories are of great interest because of their simple structure and non-volatile character. They are particularly appealing as they are good candidates for substituting flash memories. In this work, the performance of the MIM structure that takes part of a 4 kbit memory array based on 1-transistor-1-resistance (1T1R) cells was studied in terms of control of intermediate states and cycle durability. DC and small signal experiments were carried out in order to fully characterize the devices, which presented excellent multilevel capabilities and resistive-switching behavior.Ministerio de Ciencia, Innovación y Universidades (Grant, TEC2017-84321-C4-2-R )Fondos Feder y la Deutsche Forschungsgemeinschaft (German Research Foundation) ( with Project-ID 434 434 223- SFB1461)The Federal Ministry of Education and Research of Germany under (grant number 16ES1002

Study of TiN/Ti/HfO2/W resistive switching devices: characterization and modeling of the set and reset transitions using an external capacitor discharge

Producción CientíficaIn this work, we have studied the control of set and the reset transitions in TiN/Ti/HfO2/W resistive switching devices using a new approach based on the injection of a limited amount of charge through the use of a capacitor discharge. Instead of applying conventional voltage or current signals, the capacitor discharge through the devices is able to perform both transitions. An accumulative process is observed if we apply consecutive discharges, and, when increasing the capacitor voltage in each discharge, the transitions between both resistance states are completed. In addition, it has been shown that faster transitions require larger capacitor voltages. Further, the electrical results were used to tune the dynamic memdiode model, which was employed to simulate set and reset processes driven by the capacitor discharges. The model successfully reproduced the measured memristor response to the capacitor discharge.Ministerio de Ciencia, Innovación y Universidades y programa FEDER (proyectos TEC2017-84321-C4-1-R, TEC2017-84321-C4-2-R, TEC2017-84321-C4-3-R y TEC2017-84321-C4-4-R

Influences of the temperature on the electrical properties of HfO2-based resistive switching devices

Producción CientíficaIn the attempt to understand the behavior of HfO2-based resistive switching devices at low temperatures, TiN/Ti/HfO2/W metal–insulator–metal devices were fabricated; the atomic layer deposition technique was used to grow the high-k layer. After performing an electroforming process at room temperature, the device was cooled in a cryostat to carry out 100 current–voltage cycles at several temperatures ranging from the “liquid nitrogen temperature” to 350 K. The measurements showed a semiconducting behavior in high and low resistance states. In the low resistance state, a hopping conduction mechanism was obtained. The set and reset voltages increased when temperature decreased because the thermal energies for oxygen vacancies and ions were reduced. However, the temperature did not influence the power absorbed in the reset transition, indicating the local temperature in the filament controls the transition. The set transition turned from gradual to abrupt when decreasing the temperature, due to a positive feedback between the current increase and the Joule heating at low temperatures.Ministerio de Economía, Industria y Competitividad - Fondo Europeo de Desarrollo Regional (Projects TEC2017-84321-C4-2-R and TEC2017-84321-C4-1-R