First-principles investigation of carbon-based nanomaterials for supercapacitors

Supercapacitors are electrochemical energy storage devices known for their large power densities and long lifetimes yet limited energy densities. A conventional understanding of supercapacitors relates the high power to fast ion accumulation at the polarized electrode interface, forming the so-called electric double layer (EDL), and the low energy to limited electrode surface area (SA). To improve the energy density, the capacitance may be enhanced by using high SA electrode materials such as carbon-based nanomaterials. While promising results have been experimentally reported, capacitances have also been noted to exhibit a highly non-linear relationship with SA. These interesting observations suggest that a gap exists in our fundamental understanding of charge storage mechanisms in the EDL of carbon nanomaterials. Given that EDLs are typically on the order of 1-3 nm thick, theoretical simulations can elucidate these unknown physical insights in order to identify new design principles for future electrode materials. In this dissertation, we explore two broad types of carbon-based nanomaterials, which are separated into two Parts, using a combined density functional theory and classical molecular dynamics computational approach. In Part I, we study the capacitance using various chemically and/or structurally modified graphene (or graphene-derived) materials which is motivated by previous accounts of the limited capacitance using pristine graphene. Our analysis demonstrates the viability of dramatically improving the capacitance using graphene-derived materials owing to enhancements in the quantum capacitance with marginal effects on the double layer capacitance. In Part II, we investigate the capacitance using nanoporous carbon materials which is motivated by experimental observations that relate capacitance to pore width rather than SA. Our findings confirm that promoting ion confinement through pore width control can enhance capacitance, but also identify pore shape dispersity as another important structural feature that facilitates fast ion dynamics during charging/discharging. The work in this dissertation presents an overview of new insights into charge storage mechanisms using low-dimensional carbon-based nanomaterials and future directions for materials development. Moreover, we anticipate that the established methodologies and analyses can be broadly applicable to the study of other applications utilizing electrified interfaces, including capacitive deionization and liquid-gated field effect transistors.Chemical Engineerin

A Touchscreen Assay to Probe the Role of the Serotonergic System in Learning and Visual Information Processing

The neurotransmitter serotonin is involved in numerous processes in the brain such as behavior, learning, memory, mood, and neurodevelopment. Serotonin signaling is regulated by the serotonin transporter protein (SERT), which maintains normal serotonin levels. Mutations in the SERT gene are known to correlate with cognitive and behavioral deficits seen in psychiatric disorders, such as anxiety disorders, depression, and autism spectrum disorder. Researchers study these deficits using SERT knockout (KO) mice, a model that lacks functional SERT and displays changes in anxiety, learning, and motivation. We are interested in how the absence of SERT affects visual processing and learning. A popular method of evaluating learning in mice is a touchscreen-based learning paradigm, similar to tests used with both humans and primate models. We have applied this paradigm in our laboratory to study the effect of SERT KO on learning and the visual discrimination of global motion. Mice were first taught to select a visual stimulus for a food reward, then trained to discriminate between left and right coherent dot motion. Our results demonstrate that mice can learn to discriminate between different types of visual stimuli, giving us an experimental platform for future studies of learning and perception in SERT KO mice

Active Transport of Peptides Across the Intact Human Tympanic Membrane.

We previously identified peptides that are actively transported across the intact tympanic membrane (TM) of rats with infected middle ears. To assess the possibility that this transport would also occur across the human TM, we first developed and validated an assay to evaluate transport in vitro using fragments of the TM. Using this assay, we demonstrated the ability of phage bearing a TM-transiting peptide to cross freshly dissected TM fragments from infected rats or from uninfected rats, guinea pigs and rabbits. We then evaluated transport across fragments of the human TM that were discarded during otologic surgery. Human trans-TM transport was similar to that seen in the animal species. Finally, we found that free peptide, unconnected to phage, was transported across the TM at a rate comparable to that seen for peptide-bearing phage. These studies provide evidence supporting the concept of peptide-mediated drug delivery across the intact TM and into the middle ears of patients

Azimuthal anisotropy: transition from hydrodynamic flow to jet suppression

Measured 2nd and 4th azimuthal anisotropy coefficients v_{2,4}(N_{part}), p_T) are scaled with the initial eccentricity \varepsilon_{2,4}(N_{part}) of the collision zone and studied as a function of the number of participants N_{part} and the transverse momenta p_T. Scaling violations are observed for p_T \alt 3 GeV/c, consistent with a $p_T^2$ dependence of viscous corrections and a linear increase of the relaxation time with $p_T$. These empirical viscous corrections to flow and the thermal distribution function at freeze-out constrain estimates of the specific viscosity and the freeze-out temperature for two different models for the initial collision geometry. The apparent viscous corrections exhibit a sharp maximum for p_T \agt 3 GeV/c, suggesting a breakdown of the hydrodynamic ansatz and the onset of a change from flow-driven to suppression-driven anisotropy.Comment: 5 pages, 4 figs; submitted for publicatio

Constraints on models for the initial collision geometry in ultra relativistic heavy ion collisions

Monte Carlo (MC) simulations are used to compute the centrality dependence of the collision zone eccentricities ($\epsilon_{2,4}$), for both spherical and deformed ground state nuclei, for different model scenarios. Sizable model dependent differences are observed. They indicate that measurements of the $2^{\text{nd}}$ and $4^{\text{th}}$ order Fourier flow coefficients $v_{2,4}$, expressed as the ratio $\frac{v_4}{(v_2)^2}$, can provide robust constraints for distinguishing between different theoretical models for the initial-state eccentricity. Such constraints could remove one of the largest impediments to a more precise determination of the specific viscosity from precision $v_{2,4}$ measurements at the Relativistic Heavy Ion Collider (RHIC).Comment: 4 pages, 3 figs - version accepted for publicatio

Using Pupillometry to Characterize Visual Perception in Autistic Mouse Models

Fragile X syndrome (FXS) is the leading genetic cause of autism. Individuals with Fragile X Syndrome (FXS) commonly display social, behavioral, and intellectual disabilities. Perceptual deficits and their underlying neural activity remain poorly characterized in FXS and other autism spectrum disorders (ASD’s). To explore visual perception in autism, we developed camera based pupil tracking software using OpenCV (an open-source computer vision library) capable of measuring visually evoked changes in pupil area and position in the FXS mouse model (Fmr1 KO). Changes in pupil area and position are believed to correlate with changes in arousal or visual processing and may serve as an indirect readout of brain state. To explore visually evoked changes in pupil area, head-restrained wild type or Fragile X mice were exposed to visual stimulation consisting of sinusoidal gratings. The average pupil area of Fragile X mice was increased compared to wild type controls. Our results suggest that online pupillometry has a high potential to serve as a diagnostic tool for autism spectrum disorders