Jaeseok Yi

Vertical Pillar-Superlattice Array and Graphene Hybrid Light Emitting Diodes

We report a type of device that combines vertical arrays of one-dimensional (1D) pillar-superlattice (PSL) structures with 2D graphene sheets to yield a class of light emitting diode (LED) with interesting mechanical, optical, and electrical characteristics. In this application, graphene sheets coated with very thin metal layers exhibit good mechanical and electrical properties and an ability to mount, in a freely suspended configuration, on the PSL arrays as a top window electrode. Optical characterization demonstrates that graphene exhibits excellent optical transparency even after deposition of the thin metal films. Thermal annealing of the graphene/metal (Gr/M) contact to the GaAs decreases the contact resistance, to provide enhanced carrier injection. The resulting PSL-Gr/M LEDs exhibit bright light emission over large areas. The result suggests the utility of graphene-based materials as electrodes in devices with unusual, nonplanar 3D architectures.

ZnO nanorods-graphene hybrid structures for enhanced current spreading and light extraction in GaN-based light emitting diodes

One-dimensional and two-dimensional hybrid structures, composed of vertical ZnO nanorods grown on large-area graphene, are successfully integrated onto the GaN/InGaN light emitting diodes (LEDs). Compared with GaN LED without transparent conducting electrode, current injection and light emission increased almost 2–3 times, respectively, by the introduction of graphene based conducting electrode. Additional ∼66% increase in light emission was achieved by growing the ZnO nanorods on the graphene, which is consistent with the finite difference time domain modeling result. Furthermore, electroluminescence intensity profiles confirm the uniform light emission with high brightness in GaN LED with the ZnO nanorods-graphene hybrid electrode.

Engineering electronic properties of graphene by coupling with Si-Rich, two-dimensional Islands

Recent theoretical and experimental studies demonstrated that breaking of the sublattice symmetry in graphene produces an energy gap at the former Dirac point. We describe the synthesis of graphene sheets decorated with ultrathin, Si-rich two-dimensional (2D) islands (i.e., Gr:Si sheets), in which the electronic property of graphene is modulated by coupling with the Si-islands. Analyses based on transmission electron microscopy, atomic force microscopy, and electron and optical spectroscopies confirmed that Si-islands with thicknesses of ~2 to 4 nm and a lateral size of several tens of nm were bonded to graphene via van der Waals interactions. Field-effect transistors (FETs) based on Gr:Si sheets exhibited enhanced transconductance and maximum-to-minimum current level compared to bare-graphene FETs, and their magnitudes gradually increased with increasing coverage of Si layers on the graphene. The temperature dependent current-voltage measurements of the Gr:Si sheet showed approximately a 2-fold increase in the resistance by decreasing the temperature from 250 to 10 K, which confirmed the opening of the substantial bandgap (~2.5-3.2 meV) in graphene by coupling with Si islands.

Simple, Large-Scale Patterning of Hydrophobic ZnO Nanorod Arrays

Here we describe a simple, versatile technique to produce large-scale arrays of highly ordered ZnO nanorods. Patterning of three distinct ZnO crystal morphologies is demonstrated through use of different ZnO seed layers. Array formation is accomplished through a simple variation on nanosphere lithography that imprints a thickness variation across a PMMA mask layer. The area of exposed seed layer is controlled through etching time in an oxygen plasma. Subsequent hydrothermal growth from the patterned seed layer produces high-quality ZnO crystals in uniform arrays. The high uniformity of the patterned array is shown to induce a high contact angle hydrophobic state even without the need for chemical modification of the ZnO surface. This technique provides a straightforward way to integrate the optical and electrical properties of high-quality ZnO nanorods with the tunable fluidic properties at the surface of well-ordered arrays.

Synthesis of ZnO nanotubes and nanotube-nanorod hybrid hexagonal networks using a hexagonally close-packed colloidal monolayer template

We present a new synthetic approach, via hydrothermal process with the use of polystyrene (PS) colloids, to fabricate vertically aligned, single crystalline ZnO nanotube arrays. Electron microscopy images revealed that single crystalline nanotubes with inner diameters of ∼15–20 nm and wall thicknesses of ∼10–15 nm were formed just below the PS colloids, whereas solid nanorods were grown in the absence of PS colloids. In addition, nanorods enclosing the PS colloids exhibited much faster growth rates than those on the area not covered with PS colloids. These results indicate that the introduction of PS colloids affected the formation and diffusion of adatoms. The growth behavior of ZnO crystals with regards to the PS colloids was exploited to convert the ZnO nanostructures from solid to nanotube-nanorod hybrid networks by introducing hexagonally close-packed PS colloidal monolayers. Moreover, we demonstrated further conversion to complete tubular forms by reducing the aperture size between adjacent PS colloids with thermal annealing.

Reversible and Irreversible Responses of Defect-Engineered Graphene-Based Electrolyte-Gated pH Sensors

We have studied the role of defects in electrolyte-gated graphene mesh (GM) field-effect transistors (FETs) by introducing engineered edge defects in graphene (Gr) channels. Compared with Gr-FETs, GM-FETs were characterized as having large increments of Dirac point shift (∼30-100 mV/pH) that even sometimes exceeded the Nernst limit (59 mV/pH) by means of electrostatic gating of H(+) ions. This feature was attributed to the defect-mediated chemisorptions of H(+) ions to the graphene edge, as supported by Raman measurements and observed cycling characteristics of the GM FETs. Although the H(+) ion binding to the defects increased the device response to pH change, this binding was found to be irreversible. However, the irreversible component showed relatively fast decay, almost disappearing after 5 cycles of exposure to solutions of decreasing pH value from 8.25 to 6.55. Similar behavior could be found in the Gr-FET, but the irreversible component of the response was much smaller. Finally, after complete passivation of the defects, both Gr-FETs and GM-FETs exhibited only reversible response to pH change, with similar magnitude in the range of 6-8 mV/pH.

Bioinspired morphogenesis of highly intricate and symmetric silica nanostructures.

Biosilification is of interest due to its capability to produce a highly intricate structure under environmentally friendly conditions. Despite the considerable effort that has been devoted toward biomimetic silification, the synthesis of highly complex silica structures, as found in the structures of diatom cell walls, is still in its infancy. Here, we report the bioinspired fabrication of well-organized and symmetric silica nanostructured networks, involving phase separation and silicic acid polymerization processes, in analogy to the morphogenesis of diatom cell walls. Our approach exploits self-assembled silica spheres as a self-source of the silicic acids as well as scaffolds that, interplayed with droplets of ammonium hexafluorosilicate, direct the site-specific silification. Moreover, we have achieved multiple morphological evolutions with subtle changes in the process, which demonstrates exquisite levels of control over silica morphogenesis.

Graphene meshes decorated with palladium nanoparticles for hydrogen detection

We fabricated flexible and transparent hydrogen gas sensors based on a palladium-decorated graphene mesh. Electron microscopy analysis confirmed that ~2–3 nm diameter Pd nanoparticles were uniformly dispersed on the graphene mesh surfaces. The sensors were highly transparent with an average optical transmission of >90% to visible light and they exhibited good electrical stability with a resistance change of 1.5% under a tensile strain of 1.1% in a cyclic bending-unbending test. Compared with graphene-based sensors, the graphene mesh sensors exhibited a faster response to hydrogen gas with sensitivity as high as ~5% at a low concentration of 10 ppm H2/air, even at room temperature. The enhanced H2 detection characteristic of the graphene mesh sensors is attributed to the existence of edges.

Chemical and biological sensors based on defect-engineered graphene mesh field-effect transistors

Graphene has been intensively studied for applications to high-performance sensors, but the sensing characteristics of graphene devices have varied from case to case, and the sensing mechanism has not been satisfactorily determined thus far. In this review, we describe recent progress in engineering of the defects in graphene grown by a silica-assisted chemical vapor deposition technique and elucidate the effect of the defects upon the electrical response of graphene sensors. This review provides guidelines for engineering and/or passivating defects to improve sensor performance and reliability.

Site-specific synthesis of ZnO nanocrystalline networks via a hydrothermal method

ZnO nanocrystalline networks (NCNWs) consisting of percolating nanocrystals with irregular shape and size were synthesized using Al seed layers in a hydrothermal process. Various thicknesses of Al films were used to assess the effects of film thickness on the formation of ZnO NCNWs; the coverage and size of the ZnO nanocrystals increased with an increasing Al film thickness. In addition, by exploiting the seed layer-dependent crystal growth behaviors, two distinctly different ZnO nanostructures, nanorods on ZnO seed and NCNWs on Al seed, could be selectively achieved on the same substrate under the same growth conditions. Spectrally- and spatially-resolved investigations of these two ZnO nanostructures were performed using cathodoluminescence, which provided a significant opportunity to study the effect of the nanostructures on the luminescent characteristics. The ZnO NCNWs have an extremely high surface to volume ratio and sufficient inter-space, which enabled the conversion of the surface property from hydrophilic to superhydrophobic.