Hyobin Yoo

Imaging the Three-Dimensional Crust of the Korean Peninsula by Joint Inversion of Surface-Wave Dispersion and Teleseismic Receiver Functions

Abstract A detailed study of the 3D variation of shear-wave velocities in thesouthern part of the Korean Peninsula is made by combining high-frequency surface-wave tomography results of Cho et al. (2006b) with teleseismic P -wave receiverfunctions at 80 locations on the peninsula. Receiver functions were derived fromhigh-gain acceleration, short-period, and broadband digital data streams of the KoreaMeteorological Administration ( KMA ) and Korean Institute for Geosciences andMineral Resources ( KIGAM ) networks. Vertical cross sections trace the lateral vari-ation in the depth to the Moho, the variation of low velocities near the surface, andthe variable thickness of the transition from surface velocities to midcrustal veloci-ties. The derived crustal structure provides new insights on the evolution of theKorean crust.Introduction Geophysics, and seismology, in particular, provides in-direct means for assessing the larger- to local-scale, present-day architecture of continental masses through remote sens-ing (Bostock, 1999). Although field geology has providedvast information on the upper crust, mapping variations instructure in depth through geophysical investigation is aprincipal step in unraveling and understanding the evolutionof the continental lithosphere. In an area whose genetic pro-cess is relatively unknown, such as the Korean Peninsula, itis important to add geophysical constraints to reveal the con-tinental evolution.The Korean Peninsula (Fig. 1), situated at the easternmargin of the Eurasian continent, is a tectonic assemblageof two Phanerozoic mobile belts, the Imjingang belt and theOkcheon (fold) belt; three Precambrian basement terrains,Nangrim, Gyeonggi, and Yeongnam Massifs from north tosouth; and one volcanoclastic basin, that is, Gyeongsang ba-sin (Reedmann and Um, 1975; Cluzel

Position- and morphology-controlled ZnO nanostructures grown on graphene layers.

Hybrid heterostructures of low-dimensional semiconductor nanostructures with two-dimensional (2-D) graphene layers are emerging as new materials for fabricating transferable and/ or fl exible optoelectronic and electronic devices. [ 1–6 ] The semiconductor nanostructures work as effi cient channels for carrier transport and electrical pumping to radiative recombination, thereby improving the device performances greatly in optoelectronics and electronics. Furthermore, the graphene layers, which have excellent electrical and thermal conductivity, high mechanical strength and elasticity, and/or optical transparency, act as novel substrates offering new functionalities such as transferability or fl exibility. [ 7–13 ] In those previous reports, however, the nanostructures were randomly formed on graphene layers since nucleation sites on the graphene layers could not be controlled, preventing practical manipulation of individual nanostructure. It is well known that control of positions and morphology of nanostructures is a prerequisite for the nanostructure to be exploited as building blocks for fabricating various types of nanodevices. [ 14–17 ] Accordingly, the position and morphology control of the low-dimensional nanostructure on the graphene layers is crucial for practical applications. The well-controlled nanostructure on the graphene layers could serve as templates for achieving heteroepitaxy of ultrathin layer on the surface, yielding various quantum heterostructures for diverse applications including optoelectronics, electronics, and photovoltaics. Furthermore, precise position control of the individual nanostructures is a great advantage for realizing the integration of semiconductor nanodevices with graphene electronics for fl exible and/or transparent display applications. Here, we report the positionand morphologycontrolled growth of ZnO nanostructures using artifi cially formed graphene step-edges. Then, the resulting structural and optical properties of the ZnO nanostructure are also discussed. Furthermore, in order to demonstrate the feasibility of those

Growth and characterizations of GaN micro-rods on graphene films for flexible light emitting diodes

We report the growth of GaN micro-rods and coaxial quantum-well heterostructures on graphene films, together with structural and optical characterization, for applications in flexible optical devices. Graphene films were grown on Cu foil by means of chemical vapor deposition, and used as the substrates for the growth of the GaN micro-rods, which were subsequently transferred onto SiO2/Si substrates. Highly Si-doped, n-type GaN micro-rods were grown on the graphene films using metal–organic chemical vapor deposition. The growth and vertical alignment of the GaN micro-rods, which is a critical factor for the fabrication of high-performance light-emitting diodes (LEDs), were characterized using electron microscopy and X-ray diffraction. The GaN micro-rods exhibited promising photoluminescence characteristics for optoelectronic device applications, including room-temperature stimulated emission. To fabricate flexible LEDs, InxGa1–xN/GaN multiple quantum wells and a p-type GaN layer were deposited coaxially on the GaN micro-rods, and transferred onto Ag-coated polymer substrates using lift-off. Ti/Au and Ni/Au metal layers were formed to provide electrical contacts to the n-type and p-type GaN regions, respectively. The micro-rod LEDs exhibited intense emission of visible light, even after transfer onto the flexible polymer substrate, and reliable operation was achieved following numerous cycles of mechanical deformation.

Microstructures of GaN Thin Films Grown on Graphene Layers

Plan-view and cross-sectional transmission electron microscopy images show the microstructural properties of GaN thin films grown on graphene layers, including dislocation types and density, crystalline orientation and grain boundaries. The roles of ZnO nanowalls and GaN intermediate layers in the heteroepitaxial growth of GaN on graphene, revealed by cross-sectional transmission electron microscopy, are also discussed.

Microstructural defects in GaN thin films grown on chemically vapor- deposited graphene layers

Microstructural defects in GaN thin films grown on graphene produced via chemical vapor deposition have been investigated using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD analysis reveals the preferred orientations of the GaN films. We further examined the microstructural defects such as grain boundaries and threading dislocations present in the films using TEM. Plan-view TEM analysis showed presence of both high- and low-angle grain boundaries and the threading dislocations mostly bound to those grain boundaries. Moreover, the characteristics and behavior of the threading dislocations were also investigated using cross-section TEM analysis.

Flexible GaN Light-Emitting Diodes Using GaN Microdisks Epitaxial Laterally Overgrown on Graphene Dots

The epitaxial lateral overgrowth (ELOG) of GaN microdisks on graphene microdots and the fabrication of flexible light-emitting diodes (LEDs) using these microdisks is reported. An ELOG technique with only patterned graphene microdots is used, without any growth mask. The discrete micro-LED arrays are transferred onto Cu foil by a simple lift-off technique, which works reliably under various bending conditions.

High‐Resolution Observation of Nucleation and Growth Behavior of Nanomaterials Using a Graphene Template

By using graphene as an electron beam-transparent substrate for both nanomaterial growth and transmission electron microscopy (TEM) measurements, we investigate initial growth behavior of nanomaterials. The direct growth and imaging method using graphene facilitate atomic-resolution imaging of nanomaterials at the very early stage of growth. This enables the observation of the transition in crystal structure of ZnO nuclei and the formation of various defects during nanomaterial growth.

GaN light-emitting diodes on glass substrates with enhanced electroluminescence

We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates by controlling GaN crystal morphology, crystallinity, and device fabrication. Depending on the degree of epitaxy, we studied three different GaN morphologies; randomly oriented GaN polycrystals, nearly single-crystalline pyramid arrays, and fully single-crystalline pyramid arrays, which were fabricated by controlling the epitaxial relationship with the substrates. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. All the different GaN morphologies were further fabricated into LEDs to investigate their EL characteristics. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of about 2700 and 1150 cd m−2 at peak wavelengths of 537 and 480 nm, respectively.

Heterointerface effects in the electrointercalation of van der Waals heterostructures

Molecular-scale manipulation of electronic and ionic charge accumulation in materials is the backbone of electrochemical energy storage1–4. Layered van der Waals (vdW) crystals are a diverse family of materials into which mobile ions can electrochemically intercalate into the interlamellar gaps of the host atomic lattice5,6. The structural diversity of such materials enables the interfacial properties of composites to be optimized to improve ion intercalation for energy storage and electronic devices7–12. However, the ability of heterolayers to modify intercalation reactions, and their role at the atomic level, are yet to be elucidated. Here we demonstrate the electrointercalation of lithium at the level of individual atomic interfaces of dissimilar vdW layers. Electrochemical devices based on vdW heterostructures13 of stacked hexagonal boron nitride, graphene and molybdenum dichalcogenide (MoX2; X = S, Se) layers are constructed. We use transmission electron microscopy, in situ magnetoresistance and optical spectroscopy techniques, as well as low-temperature quantum magneto-oscillation measurements and ab initio calculations, to resolve the intermediate stages of lithium intercalation at heterointerfaces. The formation of vdW heterointerfaces between graphene and MoX2 results in a more than tenfold greater accumulation of charge in MoX2 when compared to MoX2/MoX2 homointerfaces, while enforcing a more negative intercalation potential than that of bulk MoX2 by at least 0.5 V. Beyond energy storage, our combined experimental and computational methodology for manipulating and characterizing the electrochemical behaviour of layered systems opens new pathways to control the charge density in two-dimensional electronic and optoelectronic devices.The electrointercalation of lithium into van der Waals heterostructures of graphene, hexagonal boron nitride and molybdenum dichalcogenides is studied at the level of individual atomic interfaces.

Latent Order in High-Angle Grain Boundary of GaN

We report the existence of latent order during core relaxation in the high-angle grain boundaries (GBs) of GaN films using atomic-resolution scanning transmission electron microscopy and ab initio density functional theory calculations. Core structures in the high-angle GBs are characterized by two pairs of Ga-N bonds located next to each other. The core type correlates strongly with the bond angle differences. We identify an order of core relaxation hidden in the high-angle GBs by further classifying the 5/7 atom cores into a stable 5/7 core (5/7(S)) and a metastable 5/7 core (5/7(M)). This core-type classification indicates that metastable cores can exist at real high-angle GBs under certain circumstances. Interestingly, 5/7(M) exhibits distinct defect states compared to 5/7(S), despite their similar atomic configurations. We investigate the reconstruction of defect states observed in 5/7(M) by analyzing the real-space wave functions. An inversion occurred between two localized states during the transition from 5/7(S) to 5/7(M). We suggest an inversion mechanism to explain the formation of new defect states in 5/7(M).