Hak Ki Yu

Ultrafast low-energy electron diffraction in transmission resolves polymer/graphene superstructure dynamics.

Probing interfaces with electrons When molecules move on surfaces, they behave differently from when inside a solid. But surface layers give off limited signals, so to probe these systems, scientists need to act fast. Gulde et al. developed an ultrafast low-energy electron diffraction technique and used it to study how a polymer moved and melted on a graphene substrate (see the Perspective by Nibbering). After hitting the sample with a laser pulse, energy transferred across the graphene-polymer interface, the polymer film became less orderly, and an amorphous phase appeared. Science, this issue p. 200; see also p. 137 Time-resolved low-energy electron diffraction resolves picosecond structural dynamics in a polymer-graphene bilayer. [Also see Perspective by Nibbering] Two-dimensional systems such as surfaces and molecular monolayers exhibit a multitude of intriguing phases and complex transitions. Ultrafast structural probing of such systems offers direct time-domain information on internal interactions and couplings to a substrate or bulk support. We have developed ultrafast low-energy electron diffraction and investigate in transmission the structural relaxation in a polymer/graphene bilayer system excited out of equilibrium. The laser-pump/electron-probe scheme resolves the ultrafast melting of a polymer superstructure consisting of folded-chain crystals registered to a free-standing graphene substrate. We extract the time scales of energy transfer across the bilayer interface, the loss of superstructure order, and the appearance of an amorphous phase with short-range correlations. The high surface sensitivity makes this experimental approach suitable for numerous problems in ultrafast surface science.

Three-Dimensional Nanobranched Indium–Tin-Oxide Anode for Organic Solar Cells

A nanostructured three-dimensional (3D) electrode using transparent conducting oxide (TCO) is an effective approach for increasing the efficiency of optoelectronic devices used in daily life. Tin-doped indium oxide (ITO) is a representative TCO with high conductivity and a high work function for anode applications. This paper reports the fabrication of a large-area ITO nanostructure with a branch shape using an electron beam evaporation process at temperatures as low as 80 °C, which was free of any carrier gas and catalyst. The large surface to volume ratio in the anode by the ITO nanobranches increases both the hole mobility by a 3D pathway and light absorbance by scattering, resulting in organic solar cells with a 12% increase in photocurrent and 20% photoconversion efficiency based on the bulk heterojunction of P3HT [region-regular poly(3-hexylthiophene)] and PCBM [phenyl-C61-butyric acid methyl ester].

Enhanced Light Out‐Coupling of Organic Light‐Emitting Diodes: Spontaneously Formed Nanofacet‐Structured MgO as a Refractive Index Modulation Layer

Organic light-emitting diodes (OLEDs) have attracted attention owing to their potential applications in full color fl at panel displays, as a back-lighting source for liquid-crystal displays. in fl exible display devices, and in solid-state lighting. [ 1–3 ] Typical bottom-emitting OLEDs are composed of a glass substrate, a transparent indium-tin-oxide (ITO) anode, thin organic multilayers, and a refl ective metal cathode. [ 4 ] The internal quantum effi ciency and lifetime have seen dramatic improvements since reports of organic fl uorescent OLEDs, and devices using phosphorescent emitting materials are operating at nearly 100% internal quantum effi ciencies. [ 5 ] However, the majority of the light generated in the organic material is confi ned in the ITO anode and glass substrate due to the large difference in the refractive indices n between their layers ( n ITO = 1.9, n glass = 1.5). [ 2 ] As explained by classical ray optics theory (i.e., Snell’s law), this results in out-coupling effi ciencies ( η out ) – expressed as the ratio of surface emission to all emitted light – of only around 20%. [ 6,7 ] The remaining 80% of the photons are trapped in the organic and substrate layers. This low out-coupling effi ciency has become one of the main limitations to highly effi cient OLEDs. Hence, the greatest potential for a substantial increase in external quantum effi ciency and power effi ciency is to enhance the light out-coupling effi ciency of OLEDs. Many techniques, such as microlenses on the glass substrate, photonic crystals, high refractive index substrates, low-index grids, and low-index-silica aerogels, have been studied to enhance η out . [ 6 , 8–12 ] Although these methods can increase the η out , they have several limitations, such as a shifted output spectra, changes in the electrical properties, complex processing techniques, and high-cost fabrication procedures. [ 13 , 14 ]

Chemical vapor deposition of graphene on a “peeled-off” epitaxial Cu(111) foil: A simple approach to improved properties.

We present a simple approach to improving the quality of CVD grown graphene, exploiting a Cu(111) foil catalyst. The catalyst is epitaxially grown by evaporation on a single crystal sapphire substrate, thickened by electroplating, and peeled off. The exposed surface is atomically flat, easily reduced, and exclusively of (111) orientation. Graphene grown on this catalyst under atmospheric CVD conditions and without wet chemical prereduction produces single crystal domain sizes of several hundred micrometers in samples that are many centimeters in size. The graphene produced in this way can easily be transferred to other substrates using well-established techniques. We report mobilities extracted using field-effect (as high as 29 000 cm(2) V(-1) s(-1)) and Hall bar measurement (up to 10 100 cm(2) V(-1) s(-1)).

Highly efficient organic light-emitting diodes with hole injection layer of transition metal oxides

We report on the advantage of interlayers using transition-metal oxides, such as iridium oxide (IrOx) and ruthenium oxide (RuOx), between indium tin oxide (ITO) anodes and 4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl (α-NPD) hole transport layers on the electrical and optical properties of organic light-emitting diodes (OLEDs). The operation voltage at a current density of 100mA∕cm2 decreased from 17to11V for OLEDs with 3-nm-thick IrOx interlayers and from 17to14V for OLEDs with 2-nm-thick RuOx ones. The maximum luminance value increased about 50% in OLED using IrOx and 108% in OLED using RuOx. Synchrotron radiation photoelectron spectroscopy results revealed that core levels of Ru 3d and Ir 4f shifted to high binding energies and that the valence band was splitting from metallic Fermi level as the surface of the transition metal was treated with O2 plasma. This provides evidence that the transition-metal surface transformed to a transition-metal oxide. The surface of the transition metal became smoother ...

Effects of Ni cladding layers on suppression of Ag agglomeration in Ag-based Ohmic contacts on p-GaN

We investigate effects of Ni cladding layers on suppression of Ag agglomeration in Ag contacts on p-GaN using high-resolution x-ray diffraction. In the annealed Ag contact, Ag (100) grains disappear and agglomerate to form a selectively epitaxial growth of Ag (111). An ultrathin Ni contact layer (10 A) below Ag film plays a role to epitaxially grow (111) Ag films on GaN, leading to the suppression of Ag agglomeration. A 20-A-thick Ni overlayer effectively acts as a passivation layer to prevent the surface diffusion of Ag atoms during annealing, leading to high light reflectance and low contact resistivity.

The role of reflective p-contacts in the enhancement of light extraction in nanotextured vertical InGaN light-emitting diodes

We report effective methods for improving light extraction efficiency for n-side-up vertical InGaN light-emitting diodes (LEDs). For the LEDs with high reflectance Ag-based p-contacts, nanotexturing of the n-GaN surface using a combination of photonic crystals and photochemical etching drastically enhances the efficiency of extraction from the top surface. In contrast, the LEDs with low reflectance Au-based p-contacts show significantly less improvement through the nanotexturing. These experimental results indicate the critical role of high reflectance p-contacts as well as surface texturing in improving the light extraction efficiency of the vertical LEDs for solid-state lighting.

Effect of N2, Ar, and O2 plasma treatments on surface properties of metals

We report the effect of N2, Ar, and O2 plasma treatments on the surface properties of metals. The carbon atoms reduced more in O2 and Ar plasma than in N2 plasma due to a chemical reaction with O2 plasma and large plasma density in Ar plasma. A water contact angle decreased after the plasma treatment regardless of the kinds of plasma gas, showing the increase in the hydrophilicity in surfaces. Synchrotron radiation photoemission spectroscopy data showed that the work function increased after N2, Ar, and O2 plasma treatments in sequence. This is due to the reduction of carbon atoms and the formation of O-rich surface in O2 plasma case.

Real-Time Label-Free Direct Electronic Monitoring of Topoisomerase Enzyme Binding Kinetics on Graphene

Monolayer graphene field-effect sensors operating in liquid have been widely deployed for detecting a range of analyte species often under equilibrium conditions. Here we report on the real-time detection of the binding kinetics of the essential human enzyme, topoisomerase I interacting with substrate molecules (DNA probes) that are immobilized electrochemically on to monolayer graphene strips. By monitoring the field-effect characteristics of the graphene biosensor in real-time during the enzyme-substrate interactions, we are able to decipher the surface binding constant for the cleavage reaction step of topoisomerase I activity in a label-free manner. Moreover, an appropriate design of the capture probes allows us to distinctly follow the cleavage step of topoisomerase I functioning in real-time down to picomolar concentrations. The presented results are promising for future rapid screening of drugs that are being evaluated for regulating enzyme activity.

Growth mechanism of metal-oxide nanowires synthesized by electron beam evaporation: A self-catalytic vapor-liquid-solid process

We report the growth mechanism of metal oxide nanostructures synthesized by electron beam evaporation. The condensed electron beam can easily decompose metal oxide sources that have a high melting point, thereby creating a self-catalytic metal nanodot for the vapor-liquid-solid process. The metal oxide nanostructures can be grown at a temperature just above the melting point of the self-catalyst by dissolving oxygen. The morphology of nanostructures, such as density and uniformity, strongly depends on the surface energy and surface migration energy of the substrate. The density of the self-catalytic metal nanodots increased with decreasing surface energies of the substrate due to the perfect wetting phenomenon of the catalytic materials on the high surface energy substrate. However, the surfaces with extremely low surface energy had difficulty producing the high density of self-catalyst nanodot, due to positive line tension, which increases the contact angle to >180°. Moreover, substrates with low surface migration energy, such as single layer graphene, make nanodots agglomerate to produce a less-uniform distribution compared to those produced on multi-layer graphene with high surface migration energy.