Cheol-Woong Yang

Evidence of graphitic AB stacking order of graphite oxides.

Graphite oxide (GO) samples were prepared by a simplified Brodie method. Hydroxyl, epoxide, carboxyl, and some alkyl functional groups are present in the GO, as identified by solid-state 13C NMR, Fourier-transform infrared spectroscopy, and X-ray photoemission spectroscopy. Starting with pyrolytic graphite (interlayer separation 3.36 A), the average interlayer distance after 1 h of reaction, as determined by X-ray diffraction, increased to 5.62 A and then increased with further oxidation to 7.37 A after 24 h. A smaller signal in 13C CPMAS NMR compared to that in 13C NMR suggests that carboxyl and alkyl groups are at the edges of the flakes of graphite oxide. Other aspects of the chemical bonding were assessed from the NMR and XPS data and are discussed. AB stacking of the layers in the GO was inferred from an electron diffraction study. The elemental composition of GO prepared using this simplified Brodie method is further discussed.

Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium

Smoothing Graphene Several methods have been reported for the growth of monolayer graphene into areas large enough for integration into silicon electronics. However, the electronic properties of the graphene are often degraded by grain boundaries and wrinkles. Lee et al. (p. 286, published online 3 April) showed that flat, single crystals of monolayer graphene can be grown by chemical-vapor deposition on silicon wafers covered by a germanium layer that aligns the grains. The graphene can be dry-transferred to other substrates, and the germanium layer can be reused for further growth cycles. Wafer-scale single-crystal monolayer graphene can be repeatedly grown on a hydrogen-terminated germanium (110) surface. The uniform growth of single-crystal graphene over wafer-scale areas remains a challenge in the commercial-level manufacturability of various electronic, photonic, mechanical, and other devices based on graphene. Here, we describe wafer-scale growth of wrinkle-free single-crystal monolayer graphene on silicon wafer using a hydrogen-terminated germanium buffer layer. The anisotropic twofold symmetry of the germanium (110) surface allowed unidirectional alignment of multiple seeds, which were merged to uniform single-crystal graphene with predefined orientation. Furthermore, the weak interaction between graphene and underlying hydrogen-terminated germanium surface enabled the facile etch-free dry transfer of graphene and the recycling of the germanium substrate for continual graphene growth.

Large-scale production of aligned carbon nanotubes by the vapor phase growth method

Abstract Aligned multiwalled carbon nanotubes have been massively synthesized by the pyrolysis of iron pentacarbonyl (Fe(CO)5) and acetylene (C2H2) mixtures in a simply designed horizontal quartz tube reactor. The growth rate and the crystallinity of carbon nanotubes were enhanced by increasing the flow rate of Ar carrier gas. The growth rate, by adopting C2H2 direct bubbling, was dramatically increased compared with Ar direct bubbling, the maximum length of 2000 μm was achieved.

One-pot synthesis of core–shell-like Pt3Co nanoparticle electrocatalyst with Pt-enriched surface for oxygen reduction reaction in fuel cells

2–3 nm sized Pt3Co nanoparticles (NPs) with Pt-enriched shells on a carbon support were prepared by a one-step ultrasound polyol process on Pt(acac)2 (acac = acetylacetonate) and Co(acac)2. The ultrasound facilitates the conversion of Co(acac)2 and retards the conversion of Pt(acac)2 into NPs, which can be explained with the different vapour pressures of the metal precursors. The Pt-enriched shell structure of the NPs by this method, as opposed to the alloy-like elemental distribution of the NPs synthesized in similar conditions but without ultrasound, was evidenced by transmission electron microscopy, magnetism data, extended X-ray absorption fine structure analyses, and chemical replacement tests of Co with Ru3+. Electrochemical oxygen reduction reaction data of these NPs showed improved performance from the commercial Pt/C with the NPs with Pt-enriched shells showing the highest activity.

Intermetallic compound layer formation between Sn–3.5 mass %Ag BGA solder ball and (Cu, immersion Au/electroless Ni–P/Cu) substrate

The growth kinetics of intermetallic compound layers formed between eutectic Sn–3.5Ag BGA (ball grid array) solder and (Cu, immersion Au/electroless Ni–P/Cu) substrate by solid-state isothermal aging were examined at temperatures between 343 and 443 K for 0–100 days. In the solder joints between the Sn–Ag eutectic solder ball and Cu pads, the intermetallic compound layer was composed of two phases: Cu6Sn5 (η-phase) adjacent to the solder and Cu3Sn (ε-phase) adjacent to the copper. The layer of intermetallic on the immersion Au/electroless Ni–P/Cu substrate was composed of Ni3Sn4. As a whole, because the values of the time exponent (n) are approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by a diffusion-controlled mechanism over the temperature range studied. The growth rate of Ni3Sn4 intermetallic compound was slower than that of the total Cu–Sn(Cu6Sn5+Cn3Sn). The apparent activation energy for growth of total Cu–Sn(Cu6Sn5+Cu3Sn) and Ni3Sn4 intermetallic compound were 64.82 and 72.54 kJ mol−1, respectively.

Synthesis of single- and double-walled carbon nanotubes by catalytic decomposition of methane

Abstract Single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) are simultaneously synthesized by catalytic decomposition of CH 4 over Fe–Mo/Al 2 O 3 catalyst. High-resolution transmission electron microscopy observation shows that produced carbon materials consist of about 70% SWNTs and about 30% DWNTs. The diameters of SWNTs are in the range of 0.8–1.5 nm while the outer and inner diameters of DWNTs are in the range of 1.75–3.1 and 0.95–2.3 nm, respectively. Raman analysis indicates that the synthesized SWNTs and DWNTs have high-quality graphite structure.

Effects of deposition parameters on the crystallinity of CeO2 thin films deposited on Si(100) substrates by r.f.-magnetron sputtering

Deposition temperature, r.f.-power and seed layer deposition time were important parameters effecting the crystallinity of CeO2 thin films deposited by r.f.-magnetron sputtering on Si(100) substrates. The CeO2 (200) peak was notable for a deposition temperature above 600°C. With decreased r.f.-power and thus lower deposition rate, the intensity of the CeO2(200) peak increased. When the seed layer deposition time was less than 20 s, the CeO2(200) peak dominated. Transmission electron microscopy (TEM) diffraction revealed that the deposited CeO2 thin film had a polycrystalline structure. Annealing at 950°C in O2 atmosphere for 30 min increased and sharpened the CeO2(200) peak.

Study of ZrO2 thin films for gate oxide applications

We investigated the microstructures and electrical properties of ZrO2 films deposited by reactive dc magnetron sputtering on Si substrates for gate dielectrics applications. We observed that the refractive index value of the ZrO2 films increased with an increase in deposition powers and annealing temperatures. The ZrO2 films deposited at elevated temperatures are polycrystalline, and both the monoclinic and tetragonal phases exist in the films. Films with higher density and improved crystallinity are obtained at higher deposition temperatures. The interfacial oxide layer between ZrO2 films and Si substrates grew upon annealing in the O2 gas ambient, which is due to the oxidation of Si substrates by the diffusion of oxidizing species from O2 gas ambient. The accumulation capacitance value increased upon annealing in the N2 gas ambient due to the densification of the films, while it decreased in O2 gas ambient due to the growth of the interfacial oxide layer.

A Study on the Thermal Stabilities of the NiGe and Ni1 − x Ta x Ge Systems

The thermal stabilities of the Ni-silicide and Ni-germanide systems were compared, and that of the Ni 0.9 Ta 0.1 /Ge alloy system was also studied. Although the NiSi film had a stable low sheet resistance (R s ) in the formation temperature range from 300 to 650°C, NiGe exhibited an abrupt increase of R s from 500°C due to the severe agglomeration. The doping of the Ni film with Ta slightly improved the thermal stability of NiGe due to the formation of a Ta-rich layer on top of NiGe and subsequent reduction of the surface-free energy of NiGe with outside ambient. During the additional thermal annealing after NiGe formation, the germanide films formed from Ni-Ta alloy also exhibited slightly improved thermal stability characteristics as compared to the pure Ni-germanide system.

Epitaxial Growth of a Single-Crystal Hybridized Boron Nitride and Graphene Layer on a Wide-Band Gap Semiconductor

Vertical and lateral heterogeneous structures of two-dimensional (2D) materials have paved the way for pioneering studies on the physics and applications of 2D materials. A hybridized hexagonal boron nitride (h-BN) and graphene lateral structure, a heterogeneous 2D structure, has been fabricated on single-crystal metals or metal foils by chemical vapor deposition (CVD). However, once fabricated on metals, the h-BN/graphene lateral structures require an additional transfer process for device applications, as reported for CVD graphene grown on metal foils. Here, we demonstrate that a single-crystal h-BN/graphene lateral structure can be epitaxially grown on a wide-gap semiconductor, SiC(0001). First, a single-crystal h-BN layer with the same orientation as bulk SiC was grown on a Si-terminated SiC substrate at 850 °C using borazine molecules. Second, when heated above 1150 °C in vacuum, the h-BN layer was partially removed and, subsequently, replaced with graphene domains. Interestingly, these graphene domains possess the same orientation as the h-BN layer, resulting in a single-crystal h-BN/graphene lateral structure on a whole sample area. For temperatures above 1600 °C, the single-crystal h-BN layer was completely replaced by the single-crystal graphene layer. The crystalline structure, electronic band structure, and atomic structure of the h-BN/graphene lateral structure were studied by using low energy electron diffraction, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy, respectively. The h-BN/graphene lateral structure fabricated on a wide-gap semiconductor substrate can be directly applied to devices without a further transfer process, as reported for epitaxial graphene on a SiC substrate.