Chalcogenide Glass-on-Graphene Photonics

Two-dimensional (2-D) materials are of tremendous interest to integrated photonics given their singular optical characteristics spanning light emission, modulation, saturable absorption, and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. In this paper, we present a new route for 2-D material integration with planar photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material which can be directly deposited and patterned on a wide variety of 2-D materials and can simultaneously function as the light guiding medium, a gate dielectric, and a passivation layer for 2-D materials. Besides claiming improved fabrication yield and throughput compared to the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light-matter interactions in the 2-D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as graphene-based mid-infrared (mid-IR) waveguide-integrated photodetectors and modulators

SU-8 microprobes for biomedical applications

152 p. : il.[ES]La presente tesis doctoral aborda el diseño, fabricación, encapsulado, y caracterización de microagujas de SU-8 para aplicaciones médicas. En la actualidad existe una amplia variedad de agujas para el registro, estimulación y dispensado de drogas, pero se han observado algunas limitaciones en relación a su diseño y material estructural utilizados. En este trabajo se han desarrollado microagujas basadas en la tecnología de SU-8 como alternativa a las agujas actuales. Primeramente se diseñan las agujas para cada tipo de aplicación, después se determinan los procedimientos de fabricación y finalmente se desarrollan los encapsulados para conectar la aguja miniaturizada con el exterior macroscópico. La aplicación de las agujas se ha centrado en dos campos biomédicos: 1) la monitorización de órganos tal como el riñón, y 2) el registro de la actividad neuronal, añadiendo la posibilidad de realizar dispensado de drogas de forma simultánea. El primer objetivo es crear microagujas que causen el menor daño posible en el tejido biológico. Las mediciones eléctricas que se llevan a cabo para conocer el estado real del tejido pueden resultar modificadas, debilitadas o destruidas si las células que constituyen el tejido han sido previamente dañadas. En este trabajo, se desarrollan microagujas basadas en la tecnología MEMS (micro electromechanical systems) para evitar daños profundos en el tejido y poder así realizar mediciones fidedignas. La tecnología MEMS integra elementos y dispositivos eléctricos, mecánicos y electrónicos miniaturizados, los cuales están basados en la industria consolidada de los Circuitos Integrados (IC). Generalmente, las dimensiones de los elementos basados en MEMS son de entre 1 y 100 micras y los dispositivos pueden variar entre 20 micras y 1 milímetro. Las técnicas base de esta tecnología son la deposición de materiales en láminas, la fotolitografía y el grabado. El silicio es el material más utilizado para crear los múltiples dispositivos MEMS, sin embargo, su rigidez y fragilidad ha motivado el estudio de otros materiales tales como los polímeros. En esta tesis se ha utilizado el polímero SU-8 como material estructural debido a sus propiedades favorables para la fabricación de microagujas. Además, la fabricación de microagujas con este polímero permite el uso de procesos de bajo coste. Esta fotoresina presenta una baja absorción a la luz UV, posibilitando exposiciones uniformes en función del espesor del polímero. Así, se obtienen perfiles verticales y un buen control dimensional para toda la estructura. Además, estudios recientes muestran una adecuada biocompatibilidad del polímero SU-8. El segundo objetivo es obtener la más alta relación señal-ruido posible en las mediciones eléctricas. Para ello se han integrado microelectrodos en las agujas y se ha estudiado la constitución física, la configuración espacial y los tratamientos superficiales de los mismos. Un determinado diseño para cada aplicación y la modificación de las técnicas de fabricación han dado como resultado una óptima capacidad sensora de los electrodos. Así, se ha demostrado su uso a través de la monitorización de episodios de isquemia y reperfusión en riñón de rata. En cuanto a las aplicaciones neuronales, se han registrado potenciales de acción con una amplitud de hasta 400-500 ¿V en hipocampo de rata. Además, se ha demostrado que los microelectrodos son capaces de discriminar diferentes fuentes neuronales. Todos estos resultados han demostrado la versatilidad del polímero para crear dispositivos sensores con aplicación en diversas áreas biomédicas. El último objetivo de esta tesis ha sido integrar canales microfluídicos en la aguja para poder dispensar drogas en aplicaciones neuronales y como resultado, detectar cambios en la actividad neuronal. Finalmente, se han llevado a cabo los primeros experimentos fluídicos in vivo en hipocampo de rata como prueba de concepto. Se dispensan 0.5 ¿l de una disolución de kainato y a continuación se registra un incremento en la actividad neuronal. Los resultados preliminares han demostrado la funcionalidad de la aguja para dispensar y monitorizar de forma simultánea aunque se tienen que realizar más experimentos y optimizar el protocolo experimental para verificar el buen funcionamiento de la aguja. En estos momentos, se están realizando más experimentos neuronales para llegar a establecer la tecnología desarrollada en esta tesis

Chemical and electrochemical studies of Leclanché cells

The densities of NH4CI-ZnCI2 solutions were measured at 25°C over a wide range of concentrations and a calculation procedure was derived assuming ideal mixing of solutions of NH4CI, ZnCl2, and the complex (NH4)ZnCI3 which accurately predicted the measured densities within plus/minus 0.7 %. The question of the NH4CI concentration at which the precipitate formed on discharge changes from Zn(NH3)2Cl2 to ZnCI2.4Zn(OH)2.H20 has been clarified and the free energies of formation of both products have been determined, for the first time for ZnCI2.4Zn(OH)2.H20. The zinc electrode potential was measured in solutions of ZnCl2 (0 to 17 molal) and of NH4CI (zero to saturation). The concentrations of the different species were calculated; ZnCI3 appeared to be predominant in all solutions except those with a large excess of NH4Cl. The solubility diagram of the NH4CI-ZnCI2- H20 system was determined for the fIrst time at 25°C. The three stages of the intermittent discharge of a Leclanché cell previously predicted by Tye have been observed and the duration of each stage explained on a theoretical basis. Hetaerolite was formed during intermittent discharge of cells containing the chemically prepared manganese dioxide (CMD) Faradiser M, as a chemical step following the normal reduction of the Mn02. This formation increased the positive electrode potential and regenerated the NH4CI by dissolving the Zn(NH3)2CI2 formed earlier in the discharge. This is the flIst reported observation of the regeneration of NH4CI caused by hetaerolite formation. In zinc chloride electrolyte, the discharge product appeared to be 2ZnC12.5Zn(OH)2.H2O and not ZnCI2.4Zn(OH)2.H20 as previously reported. An interruption technique has been used to study cells undergoing continuous discharges. The reverse reaction rate was negligible during the anodic zinc dissolution and no significant activation overpotential was observed for the manganese dioxide electrode. During these discharges in Leclanché electrolyte, the NH4CI concentration decreased at the zinc electrode interface reducing the activation overpotential and increasing the concentration overpotential. When the NH4CI concentration reached zero at the interface, the concentration proflle (moving boundary) moved toward the cathode. In the zinc chloride electrolyte, the ZnCl2 concentration at the negative electrode increased proportionally with the square root of the time on load until the diffusion layer intercepted the positive electrode. The Mn02 electrode potentials measured against a reference electrode, the Luggin capillary of which was inserted inside the cathode, were very similar for electrodeposited manganese dioxide (EMD) and CMDs throughout the discharge in ZnCl2 but higher for EMD than for a CMD in the Leclanché electrolyte. Towards the end of the discharge in both electrolytes, a large (70-100 mV) diffusion potential was generated in the separator region causing the cell voltage to decrease rapidly. This is the first time that this phenomenon has been reported. The main difference between the various cells in ZnCl2 electrolyte was in the magnitude of this diffusion potential which was significantly decreased by increase of volume of electrolyte in the cell. Although the cells containing EMD lasted longer than those containing CMD on continuous discharges. the specific performances (F mol-1) were very similar for all the materials. The non-uniform reduction rate distribution in the positive electrode has been calculated on the basis of measured potential differences within the mix using a new model of the electrode. The Mn02 potential-composition relationship conformed to the equation derived by Tye for all the dioxides at low degrees of reduction. Beyond about MnOOH0.4 the potential of EMD followed the equation derived assuming independent mobility of inserted protons and electrons while the potential of CMDs suggested permanent association of the inserted species. This difference, which was observed after both chemical and electrochemical reduction, is a new finding

Intracortical Neural Probes with Post-Implant Self-Deployed Electrodes for Improved Chronic Stability.

This thesis presents a new class of implantable intracortical neural probe with small recording electrodes that deploy away from a larger main shank after insertion. This concept is hypothesized to enhance the performance of the electrodes in chronic applications. Today, electrodes that can be implanted into the brain for months or years, are an irreplaceable tool for brain machine interfaces and neuroscience studies. However, these chronically implanted neural probes suffer from continuous loss of signal quality, limiting their utility. Histological studies found a sheath of scar tissue with decreased neural density forming around probe shanks as part of an ongoing chronic inflammation. This was hypothesized to contribute to the deterioration of recorded signals. The neural probes developed in this thesis are designed to deploy electrodes outside this sheath such that they interface with healthier neurons. To achieve this, an actuation mechanism based on starch-hydrogel coated microsprings was integrated into the shank of neural probes. Recording electrodes were positioned at the tip of micrometer fine and flexible needles that were attached to the springs. Before insertion, the hydrogel dehydrates, retracting the springs. After insertion, the gel rehydrates, releasing the springs, which then deploy the electrodes. The actuation mechanism functions in a one-time release fashion, triggered by contact with biological fluids at body temperature. The deployment of the electrodes occurred over the course of two hours and can be divided into three stages: For the first 20 s, the electrodes did not deploy. Within the first three minutes they deployed by roughly 100 µm (0.5 µm/s). Tor the following two hours they deployed an additional 20 µm (0.17 µm/min). The employed design supported six deploying electrodes, each at the end of a 5 µm wide and thick, and 100 µm long needle. These were attached to a shank with 290 µm width, 12 µm thickness and 3 mm length. The shanks could be inserted into the cortex of rats through an opening in the pia without breaking. The acquired waveforms indicate that some of the deployed electrodes were able to record neural action potentials.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113317/1/egertd_1.pd

Noise in electromigrated nanojunctions

Noise measurements are a probe beyond simple electronic transport that can reveal additional information about electronic correlations and inelastic processes. Here we report noise measurements in individual electromigrated nanojunctions, examining the evolution from the many channel regime to the tunneling regime, using a radio frequency technique. While we generally observe the dependence of noise on bias expected for shot noise, in approximately 12% of junction configurations we find discrete changes in the bias dependence at threshold values of the bias, consistent with electronic excitation of local vibrational modes. Moreover, with some regularity we find significant mesoscopic variation in the magnitude of the noise in particular junctions even with small changes in the accompanying conductance. In another $\sim$17% of junctions we observe pronounced asymmetries in the inferred noise magnitude as a function of bias polarity, suggesting that investigators should be concerned about current-driven ionic motion in the electrodes even at biases well below those used for deliberate electromigration.Comment: 13 pages, 3 figures. To appear in PR

Bioimpedance of soft tissue under compression

In this paper compression-dependent bioimpedance measurements of porcine spleen tissue are presented. Using a Cole–Cole model, nonlinear compositional changes in extracellular and intracellular makeup; related to a loss of fluid from the tissue, are identified during compression. Bioimpedance measurements were made using a custom tetrapolar probe and bioimpedance circuitry. As the tissue is increasingly compressed up to 50%, both intracellular and extracellular resistances increase while bulk membrane capacitance decreases. Increasing compression to 80% results in an increase in intracellular resistance and bulk membrane capacitance while extracellular resistance decreases. Tissues compressed incrementally to 80% show a decreased extracellular resistance of 32%, an increased intracellular resistance of 107%, and an increased bulk membrane capacitance of 64% compared to their uncompressed values. Intracellular resistance exhibits double asymptotic curves when plotted against the peak tissue pressure during compression, possibly indicating two distinct phases of mechanical change in the tissue during compression. Based on these findings, differing theories as to what is happening at a cellular level during high tissue compression are discussed, including the possibility of cell rupture and mass exudation of cellular material.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98622/1/0967-3334_33_6_1095.pd

Lithium molybdate-sulfur battery.

Rechargeable energy storage systems play a vital role in today’s automobile industry with the emergence of electric vehicles (EVs). In order to meet the targets set by the department of energy (DOE), there is an immediate need of new battery chemistries with higher energy density than the current Li- ion technology. Lithium–sulfur (Li–S) batteries have attracted enormous attention in the energy-storage, due to their high specific energy density of 2600 Wh kg-1 and operational voltage of 2.0 V. Despite the promising electrochemical characteristics, Li-S batteries suffer from serious technical challenges such as dissolution of polysulfides Li2Sx (3 ≤ x ≤8) in the electrolyte and the shuttling of polysulfide between the sulfur cathode and the lithium metal anode hindering cycling efficiency and life. There is also an immediate need to replace lithium metal (as the anode in Li-S batteries) with a suitable material. To improve the cyclability of Li-S battery, a novel method is described using mesoporous TiO2 to prevent the loss of active material from the sulfur cathode. Herein, the surface adsorbance of TiO2 for lithium polysulfides is used to prevent the leaking of soluble polysulfides into the electrolyte. Hence, cyclability with high specific capacity is achieved. The mesoporous TiO2 (titania) coated carbon-sulfur cathodes exhibit a retention capacity of 980 mAhg-1 over 100 cycles at C/3 rate (433 mA g -1) vs lithium metal anode. Further, pre-lithiated α-MoO3 is investigated as a state-of-the art anode material for Li-S batteries. α-MoO3 demonstrates lithiation potential of ~0.2 V with a specific capacity of ~1000 mAh g-1. Herein, α-MoO3 are synthesized by two different techniques; direct synthesis by Hot Wire CVD (HWCVD) technique and 40% H2/Ar reduction of impure MoO3. The initial specific charge capacities of these material are found to be over 1000 mAh g-1. The α-MoO3 electrodes of different morphologies are then assembled with mesoporous TiO2 coated sulfur cathode to make S-Li1.33Mo0.66O2 full cell, achieving initial capacity of 905 mAh g-1 at C/10 rate and 635 mAh g-1 at C/3 rate. Finally, a novel cell design is demonstrated, allowing manufacture of high energy density lithium molybdate-sulfur batteries in one step process. In this dissertation, high energy density cathodes based on mesoporous titania coated sulfur and pre-lithiated anodes based on α-MoO3 are developed for Li-S batteries by analyzing their electrochemical properties. Finally, these electrode materials are used to manufacture commercially viable Li-S pouch cells with \u3e300 Wh kg-1 energy density over 100 cycles as the outcome of this dissertation

Materials and neuroscience: validating tools for large-scale, high-density neural recording

Extracellular recording remains the only technique capable of measuring the activity of many neurons simultaneously with a sub-millisecond precision, in multiple brain areas, including deep structures. Nevertheless, many questions about the nature of the detected signal and the limitations/capabilities of this technique remain unanswered. The general goal of this work is to apply the methodology and concepts of materials science to answer some of the major questions surrounding extracellular recording, and thus take full advantage of this seminal technique. We start out by quantifying the effect of electrode impedance on the amplitude of measured extracellular spikes and background noise. Can we improve data quality by lowering electrode impedance? We demonstrate that if the proper recording system is used, then the impedance of a microelectrode, within the range typical of standard polytrodes (~ 0.1 to 2 MΩ), does not significantly affect a neural spike amplitude or the background noise, and therefore spike sorting. In addition to improving the performance of each electrode, increasing the number of electrodes in a single neural probe has also proven advantageous for simultaneously monitoring the activity of more neurons with better spatiotemporal resolution. How can we achieve large-scale, highdensity extracellular recordings without compromising brain tissue? Here we report the design and in vivo validation of a complementary metal–oxide–semiconductor (CMOS)-based scanning probe with 1356 electrodes arranged along approximately 8 mm of a thin shaft (50 μm thick and 100 μm wide). Additionally, given the ever-shrinking dimensions of CMOS technology, there is a drive to fabricate sub-cellular electrodes (< 10 μm). Therefore, to evaluate electrode configurations for future probe designs, several recordings from many different brain regions were performed with an ultra-dense probe containing 255 electrodes, each with a geometric area of 5 x 5 μm and a pitch of 6 μm. How can we validate neural probes with different electrode materials/configurations and different sorting algorithms? We describe a new procedure for precisely aligning two probes for in vivo “paired-recordings” such that the spiking activity of a single neuron is monitored with both a dense extracellular silicon polytrode and a juxtacellular micro-pipette. We gathered a dataset of paired-recordings, which is available online. The “ground truth” data, for which one knows exactly when a neuron in the vicinity of an extracellular probe generates an action potential, has been used for several groups to validate and quantify the performance of new algorithms to automatically detect/sort single-units