Top-down Si nanowire technology in discrete charge storage nonvolatile memory application

Ph.DDOCTOR OF PHILOSOPH

Charge transport in monolayer protected Au and Ag clusters in electrolyte environment and transport properties of a redox-active ionic liquid

This dissertation compiles interdisciplinary topics broadly in Raman spectroscopy, nanoparticle synthesis, nanoparticle electrochemistry and ionic liquids. Chapter one describes the overview on the two topics Monolayer protected gold and silver cluster synthesis and investigation of their electrochemical and electrical studies and, secondly, physicochemical and charge transport studies of ionic liquids (ILs) ranging from conventional (redox-inactive) to redoxactive ILs. At the end of the chapter the scope of this thesis is described in a problem solving based approach. Chapter two describes the principles and techniques which rationalize in the designed problems and the experiments. In chapter three, the general procedures used in preparing the experiments and the sample preparation for the voltammetric, electrochemical Raman, and Scanning Tunneling Microscopy/ Spectroscopy (STM/STS) experiments are discussed. In chapter four, results are presented in the form of the three publications and one chapter of first results. In the first publication, we have studied Au(55 nm)@SiO2 nanoparticles (NPs) on two low-index phases of gold and platinum single crystal electrodes in ClO4- and SO42- ion-containing electrolytes by both electrochemical methods and in-situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS). The blocking of the electrode with surfactants originating from the synthesis of as-prepared SHINERS NPs is shown and an efficient procedure to overcome this problem is introduced, which provides a fundamental platform for the application of SHINERS in surface electrochemistry and beyond. The method is based on a hydrogen evolution treatment of the SHINERS-NP-modified single-crystal surfaces. The reliability of our preparation strategy is demonstrated in electrochemical SHINERS experiments on the potential-controlled adsorption and phase formation of pyridine on Au(hkl) and Pt(hkl). High-quality Raman spectra on these well-defined and structurally carefully characterized single-crystal surfaces were obtained. The analysis of the characteristic A1 vibrational modes revealed perfect agreement with the interpretation of single-crystal voltammetric and chronoamperometric experiments. The study demonstrates that the SHINERS protocol developed in this work qualifies this Raman method as a pioneering approach with unique opportunities for in situ structure and reactivity studies at well-defined electrochemical solid/liquid interfaces. In the second publication, a proof-of-concept study is presented by electrochemically controlled scanning probe experiments performed on tailor-made Au particles of narrow dispersity. In particular the charge transport characteristics through chemically synthesized hexane-1-thiol and 4-pyridylbenzene-1-thiol mixed monolayer protected Au144 clusters (MPCs) by differential pulse voltammetry (DPV) and electrochemical scanning tunneling spectroscopy (EC-STS) are reported. The pyridyl groups exposed by the Au-MPCs enable their immobilization on Pt(111) substrates. By varying the humidity during their deposition, samples coated by stacks of compact monolayers of Au-MPCs or decorated with individual, laterally separated Au-MPCs are obtained. DPV experiments with stacked monolayers of Au144-MPCs and EC-STS experiments with laterally separated individual Au144-MPCs are performed both in aqueous and ionic liquid electrolytes. Lower capacitance values were observed for individual clusters compared to ensemble clusters. This trend remains the same irrespective of the composition of the electrolyte surrounding to the Au144-MPC. The resolution of the energy level spacing of single clusters however, is strongly affected by the proximity of neighboring particles. In the third publication, a redox-active ionic liquid (IL), 1-butyl-1’-heptyl-4,4’-bipyridinium bis(trifluoromethanesulfonyl) imide has been synthesized and its transport processes were investigated. The conductivity and viscosity of the IL, as well as the diffusion coefficients of its components were studied over a 50 °C wide temperature range: for the diffusivity studies, both the pulsed-gradient spin-echo (PGSE)–NMR technique and voltammetric measurements have been applied. The measured data are presented in the paper and are compared to each other. It was found that the diffusion coefficients determined by means of NMR and chronoamperometry measurements are, within the range of experimental error, equal — and they are (in accordance with other ionic liquid studies) higher than what the conductivity or viscosity measurements indicate. The results are interpreted in the light of the existing theories. The measured diffusion coefficients and bulk conductivities can be well interrelated based on the “ionicity” concept (that is, by treating the ionic liquid as a weak electrolyte). In agreement with the empirical Walden rule, a direct comparison between the measured conductivities and viscosities is also possible, for which a hole conduction model is utilized. Based on the fact that both the electrochemical and the NMR measurements yield practically the same diffusion coefficients in the system, there is no evidence that interpretations based in other redox-active IL systems on “homogeneous electron transfer” apply to the system studied here. In chapter 4.4, first results are presented on stabilization of monolayer protected silver clusters (“Ag9MSA7”, MSA= Mercpatosuccinic acid) and their electrochemical and electrospray ionisation mass spectrometry (ESI-MS) investigations. Here, it has been shown that an enhanced stability of “Ag9MSA7” MPCs can be achieved by submitting them to phase transfer into an organic solvent in the presence of sterically bulky counterions. The resulting phase transferred clusters, “Ag9MSA7(TOA)x” show a very high stability in ambient conditions that has enabled for the first time to perform voltammetric investigations. Size exclusion chromatography reveals also a minor yield of clusters with a smaller size during the synthesis of “Ag9MSA7” clusters. The electrochemical studies of the “Ag9MSA7(TOA+)x” (1.63 nm) clusters show an electrochemical gap of 1.19 V with a HOMO-LUMO gap of 0.87 eV. However, as an important result, the combination of the MS investigations with the electrochemically determined HOMO-LUMO gap strongly suggest that the real cluster sizes are larger than the empirically used formula “Ag9MSA7(TOA+)x”. In chapter five, the findings are summarized and an outlook is given of the investigated problems in the view of broader range

Development and characterization of two-dimensional metallic magnetic calorimeter arrays for the high-resolution X-ray spectroscopy

In the framework of this thesis the new two-dimensional detector array maXs30 was developed, fabricated and characterized. It is optimized for high-resolution X-ray spectroscopy in the energy range up to 30 keV and consists of 8x8 individual metallic magnetic calorimeters. These detectors combine the good energy resolution of crystal spectrometers and the large energy bandwidth of semiconductor detectors and measure the energy of each absorbed photon by the resulting temperature change. This change is converted to a change of magnetization by the paramagnetic alloy Ag:Er, which is read out by sensitive SQUID magnetometers. The detector can be mounted on the side arm of a mobile dry dilution refrigerator and a new 32 channel acquisition system has been developed for this array. The intrinsic energy resolution of the maXs30 detector was measured to be 7.9 eV (FWHM) and a resolving power of almost 3000 was achieved for X-rays with an energy of 60 keV. The complete maXs system was successfully tested during several experiments at the heavy ion storage ring ESR at the GSI, where the Lyman series of Xe53+ and the Balmer series of U89+ were investigated

Diseño y utilización de electrolitos gel para estudios electroquímicos de corrosión en superficies metálicas de geometría compleja

Mención Internacional en el título de doctorLa corrosión en uno de los problemas que mayor interés suscita en ingeniería a nivel mundial, y el estudio del comportamiento a corrosión de los materiales durante su vida útil no siempre es una tarea fácil. Los dispositivos que existen comúnmente en el mercado, diseñados para evaluar el comportamiento a corrosión de los materiales emplean electrolitos líquidos, por lo que suelen generar dificultades durante el montaje experimental cuando se intenta delimitar la superficie de ensayo, ya que existe la probabilidad de crear resquicios. Estas zonas son más vulnerables a la aparición de la corrosión y pueden enmascarar los resultados del ensayo. Unido a esta posible interferencia, se encuentran algunas otras dificultades, como el estudio de superficies irregulares, uniones o ángulos poco accesibles, pues los dispositivos usados hasta el momento generalmente están diseñados para el estudio de muestras planas o mecanizables. En la presente tesis doctoral, se ha desarrollado un electrolito gel a base de agar y glicerol, como alternativa a los electrolitos líquidos utilizados habitualmente en los ensayos electroquímicos, para evaluar el comportamiento a corrosión de los materiales y especialmente en superficies de geometría compleja. El precedente de esta investigación es una novedosa celda portátil con gel de agar como electrolito, diseñada para estudios del patrimonio cultural. A partir de esta propuesta, se decidió mejorar la adaptabilidad del electrolito mediante adiciones de plastificante. Con el objetivo de determinar la composición óptima del electrolito gel se ha realizado una caracterización de las propiedades físicas variando las concentraciones de agar y glicerol (plastificante). Una vez determinados los porcentajes de estos reactivos que conferían adaptabilidades óptimas al gel, se realizaron ensayos electroquímicos empleando diferentes técnicas y se compararon los resultados con los obtenidos para electrolitos líquidos con las mismas concentraciones de sal e iones despasivantes (Cl¯). Para comprender cómo el empleo de electrolitos gel podía afectar a los mecanismos mediante los cuales transcurre la pasivación y la corrosión, se estudiaron materiales con diferente comportamiento a corrosión, tales como el acero inoxidable, acero al carbono y galvanizado. El uso del electrolito gel también se validó en piezas con geometrías complejas y en diferentes aplicaciones de interés industrial. Además, se profundizó en la influencia de ciertos factores que podían afectar la resistencia a la corrosión acuosa de los materiales tales como la laminación en frío, la formación previa de óxidos a alta temperatura y la radiación γ. El resultado más relevante de la investigación realizada es la obtención de un electrolito gel con elevada capacidad de deformación sobre superficies metálicas de geometría compleja, con una conductividad similar a la que presentan los electrolitos líquidos tradicionales y que permite obtener resultados comparables y fiables sobre el comportamiento a corrosión de los materiales estudiados. El menor contenido de oxígeno disuelto y la menor difusividad de los cationes metálicos en el gel en comparación con el electrolito líquido parecen afectar moderadamente las reacciones catódica y anódica, disminuyendo la velocidad de corrosión, fundamentalmente en metales activos. Sin embargo, el gel permite obtener información relevante sobre el estado de actividad/pasividad de los metales. Además, el nuevo electrolito gel brinda la posibilidad de monitorizar in situ piezas y estructuras de componentes industriales en servicio.Corrosion in one of the greatest interest problems in engineering worldwide and to study the corrosion behaviour of materials during their useful life is not an easy task. Commercial devices used to evaluate the corrosion resistance of materials employ liquid electrolytes. However, different difficulties are found during the experimental cell assembly where the surface of the working electrode is delimited, since crevice generation can occur. These areas are more vulnerable to the appearance of corrosion and can mask the results of the test. In addition, these devices only allow studying flat samples, so there are some limitations to study irregular surfaces, joints and not easy accessible angles. In this doctoral thesis a gel electrolyte based on agar and glycerol was developed to evaluate the corrosion behaviour of several materials, especially on surfaces with complex geometry. The precedent of this research was a novel portable cell with agar gel, which was designed to study cultural heritage. Then, it was decided to improve the adaptability of this electrolyte through additions of plasticizer. In order to determine the optimal composition of gel electrolyte, a characterization of physical properties was carried out varying the concentrations of agar and glycerol (plasticizer). Then, electrochemical tests were performed using different techniques and gel electrolytes with the optimal adaptability. The results were compared with those obtained for liquid electrolytes with the same salt concentrations and aggressive ions (Cl¯). Moreover, materials with different corrosion behaviour such as stainless steel, carbon steel and galvanized steel were tested with the aim to understand the corrosion mechanisms in different systems. The use of gel electrolyte was also validated in pieces with complex geometries and in different applications of industrial interest. In addition, the influence of certain factors that can affect the resistance of materials such as cold rolling and oxidation at high temperature and γ radiation were investigated. The most relevant result of this research was to obtain a gel electrolyte with high deformation capacity on metal surfaces with complex geometry. The ionic conductivity of the gel electrolyte is similar to the ionic conductivity of traditional liquid electrolytes. Moreover, comparable and reliable results in terms of corrosion behaviour of the studied materials were reached. The lower content of dissolved oxygen and the lower diffusivity of the metal cations in the gel compared to liquid electrolyte seem to affect moderately the cathodic and anodic reactions mainly in active metals, which also decrease the corrosion rate. However, the gel electrolyte allows obtaining relevant information about the activity/passivity state of metals. In addition, the new gel electrolyte offers the possibility of monitoring in situ parts and structures of industrial components in service.Programa Oficial de Doctorado en Ciencia e Ingeniería de MaterialesPresidente: Fernando Pedraza Díaz.- Secretario: Juana Abenojar Buendía.- Vocal: Daniel de la Fuente Garcí