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  • [ October 10, 2019]

    Metal Halide Perovskite Solar Cell Research by Surface Science and Advanced Characterization

    | Metal halide perovskite materials and devices have become one of the most active research fields and continues to progress rapidly on various fronts. My group at OIST is making efforts to use surface science and advanced material characterization to obtain in-depth understanding about perovskite materials and devices. Instability, upscalability and lead toxicity are currently the major challenges for perovskite photovoltaic technology to move forward towards commercialization. In this talk, I will present our research progress on degradation mechanisms of perovskite materials], development of strategies to improve perovskite solar cell device stability, reduction of lead leakage using self-healing polymer-based encapsulation, upscalable fabrication of perovskite solar cells and modules, and surface science understanding of perovskite materials and stability.
  • [ October 09, 2019]

    Rare-Event Simulation of Regenerative Systems

    | Rare events occur by definition with a very small probability but are important to analyze because of potential catastrophic consequences. During this talk, we will focus on rare event for so-called regenerative processes, that are basically processes such that portions of the process are statistically independent of each other. For many complex and/or large models, simulation is the only tool at hand but requires specific implementations to get an accurate answer in a reasonable time. There are two main families of rare-event simulation techniques: importance sampling (IS) and splitting.

    This talk is mostly based on collaborative works with Peter W. Glynn and Marvin K. Nakayama.
  • [ September 26, 2019]

    Decision-Environment Co-evolutionary Game Dynamics in A Closed Loop

    | In response to the growing interest in how to efficiently influence complex systems of interacting self-interested agents, new efforts have been made to address open problems related to the analysis and control of evolutionary matrix games, with particular emphasis on applications in social, economic, biological and robotic networks. In our evolutionary game models for networks of interacting agents, we consider both the decision-making dynamics of interacting multiple populations and the evolution of the environment affecting the game payoffs. It is then possible to perform global convergence analysis for the resulting closed-loop co-evolutionary dynamics, to identify in particular under what conditions the dynamics converge to periodic orbits The goal is to build a general framework towards optimal control of evolutionary games on networks and ultimately resolving social dilemmas in large populations of interacting self-interested agents.
  • [ August 29, 2019]

    Nanocombinatorics: Multicomponent Multiplexed and Multifunctional Nanostructures

    | The presentation will first cover strategies for nanopatterning and nanolithography, with an emphasis on localized synthesis of multicomponent and multiplexed nanostructures The in-house developed dip-pen and analogous soft lithography schemes are being utilized for nanoparticle-based complex surface architectures that demonstrate exquisite control across many length-scales. Similarly, precursor-based nanopatterning approach is extended to demonstrate “nanocombinatoric” synthesis of multiplexed and multicomponent nanostructures, akin to “experimental materials genome”. Novel multicomponent nanostructures comprising wide range of composition across the Periodic Table are synthesized at the nanoscale, to explore their novel catalytic and functional behavior. Advanced scan probe, in-situ and ex-situ electron, ion and photon microscopy, 3D tomography, spectroscopy and synchrotron x-ray scattering approaches are being employed to fathom the most intricate details of the “microstructure” of nanostructures, coupled with innovative tools to validate their functional identity, localized and emergent properties. It will be argued that multifunctional nanostructures go beyond the “hype”, and present challenging yet exciting opportunities for synthesis-structure-architecture-form-function-performance relationships in complex materials systems.
  • [ July 01, 2019]

    Optimal Smooth Approximation for Quantile Matrix Factorization

    | Matrix factorization has wide applications in recommender systems and signal processing. Existing matrix factorization methods are mostly based on squared loss and aim to yield a low-rank matrix to interpret conditional sample means. However, in many real applications with extreme data, least squares cannot explain their central tendency or tail distributions, incurring undesired estimates. In this talk, we will present a general framework of quantile matrix factorization (QMF), which introduces the check loss originated from quantile regression into matrix factorization. However, the non-smoothness of the check loss has brought significant challenges to numerical computation. We propose a nearly optimal and efficient algorithm to solve QMF by extending Nesterov's optimal smooth approximation procedure to the case of matrix factorization. We theoretically show that under certain conditions, the optimal solution to the proposed smooth approximation will converge to the optimal solution to the original nonsmooth and nonconvex QMF problem, with competitive convergence rates. We will also present numerical simulations based on synthetic and real-world data to verify our theoretical findings as well as algorithm performance.
  • [ July 01, 2019]

    Coherent structures and extreme events in rotating multiphase turbulent flows

    | By using direct numerical simulations (DNS) at unprecedented resolution we study turbulence under rotation in the presence of simultaneous direct and inverse cascades. The accumulation of energy at large scale leads to the formation of vertical coherent regions with high vorticity oriented along the rotation axis. By seeding the flow with millions of inertial particles, we quantify -for the first time- the effects of those coherent vertical structures on the preferential concentration of light and heavy particles. Furthermore, we quantitatively show that extreme fluctuations, leading to deviations from a normal-distributed statistics, result from the entangled interaction of the vertical structures with the turbulent background. Finally, we present the first-ever measurement of the relative importance between Stokes drag, Coriolis force and centripetal forces along the trajectories of inertial particles. We discover that vortical coherent structures lead to unexpected diffusion properties for heavy and light particles in the directions parallel and perpendicular to the rotation axis.
  • [ June 24, 2019]

    Modeling Thermal-Hydraulic-Mechanical Processes in Enhanced or Engineered Geothermal Systems

    | In this talk, we will present a THM model/simulator (TOUGH2-EGS) and its application for simulation of EGS reservoirs. The simulator couples heat flow with geomechanics, which is able to describe complex fluid and heat flow behavior in multiphase, multi-component, geothermal reservoirs. In this model, the fully coupled fluid and heat flow-geomechanics formulation is developed from the linear elastic theory for the poro-elastic system. The rock compaction is then correlated with stress-and temperature dependent rock properties through the flow-stress coupling process. We will present several simulation examples for insight of geomechanics impacts on fluid and heat flow processes in EGS reservoirs.
  • [ June 05, 2019]

    Seeking suitable biomaterials for soft tissues

    | Biomaterial approaches to tissue repair & regeneration have been utilized to manage injury, damage and diseases of soft tissues, such as muscle and blood vessel. Seeking a suitable biomaterial for such use is very complex and highly significant. It is expected that the biomaterial is compatible and mimetic to the native tissue, liking a harmony between artificialization and nature. Our lab aims to utilize structure-properties-function relationship to seek suitable biomaterials for soft tissue repair, through utilities of both synthetic and natural material concepts. Due to softness and elasticity of the soft tissues, we focus on a synthetic elastic biodegradable polymer, polyurethane, with controlled degradation, tunable mechanical properties, blood compatibility and conductivity. In terms of tissue extracellular matrix complex, decellularized materials were investigated in our lab. Furthermore, we also developed robust elastic hydrogels for bioprinting, and bioadhesives for tissue gluing. The development of these materials may provide some hints for inspiring new generation of biomaterials.
  • [ May 30, 2019]

    Interface Engineering for 2D Materials Based Devices

    | Two-dimensional (2D) layered materials like graphene and transition metal dichalcogenides (TMDs) have been considered as promising building blocks for the next generation nanoelectronic devices, showing great potentials to extend the scaling limits existing in silicon based complementary metal-oxide-semiconductor field-effect-transistors (CMOS-FET) as well as to serve as a high mobility alternative to organic semiconductors for flexible electronic and optoelectronic devices. As one-atomic or a few atomic thin layers, the interface plays essential role in determining the performance of 2D materials based devices, such as charge injection/collection at metal/2D interfaces, charge carrier traps at the dielectric/2D interfaces, etc. Without precise control of the surface and interface properties, many 2D materials based devices will not function properly.
    In this talk, I will summarize and discuss our recent work for interface engineered 2D phosphorene and TMDCs based field-effect-transistors (FETs) and photo-transistors, through the combination of in-situ FET device evaluation and photoelectron spectroscopy investigation. We will particularly emphasize on the electron and hole doping effect on the transport properties and optoelectronic response of phosphorene devices.
  • [ May 30, 2019]

    Bioiontronic devices for interactive communication with biology

    | Our research interests and goals are dedicated to nano-bio electronics, specifically biocompatible electronics for medical implants and electrical therapy, based on bio-inspired/mimetic engineering and integrated top-down and bottom-up nanofabrication. Our research focus on “bioiontronics”, based on sophisticated ion control for interactive communication with biology. Ionic species dominate biological signaling in nature, so converting biochemical ionic signals into electronic signals and stimulating biological system with ions and biochemicals are an emerging technique in the nano-biofields.
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