Yun Goo Ro

High Density Individually Addressable Nanowire Arrays Record Intracellular Activity from Primary Rodent and Human Stem Cell Derived Neurons

We report a new hybrid integration scheme that offers for the first time a nanowire-on-lead approach, which enables independent electrical addressability, is scalable, and has superior spatial resolution in vertical nanowire arrays. The fabrication of these nanowire arrays is demonstrated to be scalable down to submicrometer site-to-site spacing and can be combined with standard integrated circuit fabrication technologies. We utilize these arrays to perform electrophysiological recordings from mouse and rat primary neurons and human induced pluripotent stem cell (hiPSC)-derived neurons, which revealed high signal-to-noise ratios and sensitivity to subthreshold postsynaptic potentials (PSPs). We measured electrical activity from rodent neurons from 8 days in vitro (DIV) to 14 DIV and from hiPSC-derived neurons at 6 weeks in vitro post culture with signal amplitudes up to 99 mV. Overall, our platform paves the way for longitudinal electrophysiological experiments on synaptic activity in human iPSC based disease models of neuronal networks, critical for understanding the mechanisms of neurological diseases and for developing drugs to treat them.

Enhanced conversion efficiency in wide-bandgap GaNP solar cells

In this work, we demonstrate ∼2.05 eV dilute nitride GaNP solar cells on GaP substrates for potential use as the top junction in dual-junction integrated cells on Si. By adding a small amount of N into indirect-bandgap GaP, GaNP has several extremely important attributes: a direct-bandgap that is also tunable, and easily attained lattice-match with Si. Our best GaNP solar cell ([N] ∼ 1.8%, Eg ∼ 2.05 eV) achieves an efficiency of 7.9%, even in the absence of a window layer. This GaNP solar cells efficiency is 3× higher than the most efficient GaP solar cell to date and higher than other solar cells with similar direct bandgap (InGaP, GaAsP). Through a systematic study of the structural, electrical, and optical properties of the device, efficient broadband optical absorption and enhanced solar cell performance are demonstrated.

Facet-Selective Nucleation and Conformal Epitaxy of Ge Shells on Si Nanowires.

Knowledge of nanoscale heteroepitaxy is continually evolving as advances in material synthesis reveal new mechanisms that have not been theoretically predicted and are different than what is known about planar structures. In addition to a wide range of potential applications, core/shell nanowire structures offer a useful template to investigate heteroepitaxy at the atomistic scale. We show that the growth of a Ge shell on a Si core can be tuned from the theoretically predicted island growth mode to a conformal, crystalline, and smooth shell by careful adjustment of growth parameters in a narrow growth window that has not been explored before. In the latter growth mode, Ge adatoms preferentially nucleate islands on the {113} facets of the Si core, which outgrow over the {220} facets. Islands on the low-energy {111} facets appear to have a nucleation delay compared to the {113} islands; however, they eventually coalesce to form a crystalline conformal shell. Synthesis of epitaxial and conformal Si/Ge/Si core/multishell structures enables us to fabricate unique cylindrical ring nanowire field-effect transistors, which we demonstrate to have steeper on/off characteristics than conventional core/shell nanowire transistors.

Atomic Scale Analysis of the Enhanced Electro- and Photo-Catalytic Activity in High-Index Faceted Porous NiO Nanowires

Catalysts play a significant role in clean renewable hydrogen fuel generation through water splitting reaction as the surface of most semiconductors proper for water splitting has poor performance for hydrogen gas evolution. The catalytic performance strongly depends on the atomic arrangement at the surface, which necessitates the correlation of the surface structure to the catalytic activity in well-controlled catalyst surfaces. Herein, we report a novel catalytic performance of simple-synthesized porous NiO nanowires (NWs) as catalyst/co-catalyst for the hydrogen evolution reaction (HER). The correlation of catalytic activity and atomic/surface structure is investigated by detailed high resolution transmission electron microscopy (HRTEM) exhibiting a strong dependence of NiO NW photo- and electrocatalytic HER performance on the density of exposed high-index-facet (HIF) atoms, which corroborates with theoretical calculations. Significantly, the optimized porous NiO NWs offer long-term electrocatalytic stability of over one day and 45 times higher photocatalytic hydrogen production compared to commercial NiO nanoparticles. Our results open new perspectives in the search for the development of structurally stable and chemically active semiconductor-based catalysts for cost-effective and efficient hydrogen fuel production at large scale.

GaP/GaNP Heterojunctions for Efficient Solar-Driven Water Oxidation

The growth and characterization of an n-GaP/i-GaNP/p+ -GaP thin film heterojunction synthesized using a gas-source molecular beam epitaxy (MBE) method, and its application for efficient solar-driven water oxidation is reported. The TiO2 /Ni passivated n-GaP/i-GaNP/p+ -GaP thin film heterojunction provides much higher photoanodic performance in 1 m KOH solution than the TiO2 /Ni-coated n-GaP substrate, leading to much lower onset potential and much higher photocurrent. There is a significant photoanodic potential shift of 764 mV at a photocurrent of 0.34 mA cm-2 , leading to an onset potential of ≈0.4 V versus reversible hydrogen electrode (RHE) at 0.34 mA cm-2 for the heterojunction. The photocurrent at the water oxidation potential (1.23 V vs RHE) is 1.46 and 7.26 mA cm-2 for the coated n-GaP and n-GaP/i-GaNP/p+ -GaP photoanodes, respectively. The passivated heterojunction offers a maximum applied bias photon-to-current efficiency (ABPE) of 1.9% while the ABPE of the coated n-GaP sample is almost zero. Furthermore, the coated n-GaP/i-GaNP/p+ -GaP heterojunction photoanode provides a broad absorption spectrum up to ≈620 nm with incident photon-to-current efficiencies (IPCEs) of over 40% from ≈400 to ≈560 nm. The high low-bias performance and broad absorption of the wide-bandgap GaP/GaNP heterojunctions render them as a promising photoanode material for tandem photoelectrochemical (PEC) cells to carry out overall solar water splitting.

Flexible inorganic light emitting diodes and transparent PEDOT:PSS/Parylene C for simultaneous optogenetics and electrocorticography (Conference Presentation)

Electrocorticography (ECoG) is a powerful tool for direct mapping of local field potentials from the brain surface. Progress in development of high-fidelity materials such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on thin conformal substrates such as parylene C enabled intimate contact with cortical surfaces and higher quality recordings from small volumes of neurons. Meanwhile, stimulation of neuronal activity is conventionally accomplished with electrical microstimulation and transcranial magnetic stimulation that can be combined with ECoG to form the basis of bidirectional neural interface. However, these stimulation mechanisms are less controlled and primitively understood on the local and cellular levels. With the advent of optogenetics, the localization and specificity of neuronal stimulation and inhibition is possible. Therefore, the development of integrated devices that can merge the sensitivity of ECoG or depth recording with optogenetic tools can lead to newer frontiers in understanding the neuronal activity. Herein, we introduce a hybrid device comprising flexible inorganic LED arrays integrated PEDOT:PSS/parylene C microelectrode arrays for high resolution bidirectional neuronal interfaces. The flexible inorganic LEDs have been developed by the metal-organic vapor phase epitaxy of position-controlled GaN microLEDs on ZnO nanostructured templates pre-grown at precise locations on a graphene layer. By transferring it onto the microelectrode arrays, it can provides the individual electrical addressability by light stimulation patterns. We will present experimental and simulation results on the optoelectronic characteristics and light activation capability of flexible microLEDs and their evaluation in vivo.