Huaichao Yang

Thickness-Dependent Morphologies of Gold on N-Layer Graphenes

We report that gold thermally deposited onto n-layer graphenes interacts differently with these substrates depending on the number layer, indicating the different surface properties of graphenes. This results in thickness-dependent morphologies of gold on n-layer graphenes, which can be used to identify and distinguish graphenes with high throughput and spatial resolution. This technique may play an important role in checking if n-layer graphenes are mixed with different layer numbers of graphene with a smaller size, which cannot be found by Raman spectra. The possible mechanisms for these observations are discussed.

Self-assembly and magnetic properties of cobalt nanoparticles

Two- and three-dimensional superlattices of passivated cobalt nanoparticles were formed by a self-assembly technique. The size and stabilization of the cobalt nanoparticles are controlled by using the combination of oleic acid and triphenylphosphine. The cobalt nanoparticles are stable for at least 90 days without oxidation at room temperature under ambient conditions. The magnetic properties of the cobalt nanoparticles in different forms are compared, which provides helpful information on the magnetostatic interaction of the nanoparticles.

Synthesis and characterization of n-octadecayl mercaptan-protected palladium nanoparticles

Long-chain n-octadecayl mercaptan (C18H37SH)-passivated palladium nanoparticles are synthesized and characterized. The palladium nanoparticles are successfully capped by n-octadecayl mercaptan. These palladium nanoparticles have the same face-centered cubic crystalline structure as Pd in the bulk phase. The size of the capped palladium nanoparticles varies in the range of 1.3–5.5 nm for various reaction conditions. These results show that the long-chain n-octadecayl mercaptan-capped palladium nanoparticles are more stable than alkanethiolate-capped Pd nanoparticles with a shorter chain. 2003 Elsevier Science B.V. All rights reserved.

Stable cobalt nanoparticles passivated with oleic acid and triphenylphosphine

Monodisperse cobalt nanoparticles (NPs) are synthesized via a high-temperature thermal decomposition method in the presence of oleic acid and triphenylphosphine. The as-synthesized NPs are stable against further deep oxidation when they are kept in heptane (C7H16). Time-dependent x-ray photoelectron spectroscopy studies indicate that the oxidation of the as-synthesized cobalt NPs by air is slow and the valence change of cobalt from the Co NP sample kept in heptane under air is not observed for 120xa0days. The cobalt NPs have β-manganese type structure (also called e-cobalt). Annealing the NPs at 500u2009°C under Ar (95%)+H2xa0(5%) converts these particles from e-Co to fcc-Co. Two-dimensional (2D) and three-dimensional (3D) self-assembled superlattices of passivated cobalt NPs are formed by slow evaporation of the carrier solvent.

Raman scattering of monolayer graphene: the temperature and oxygen doping effects

Raman spectra of monolayer graphene at various temperatures (303?473?K) are measured. In Raman scattering with wave numbers ranging from 1200 to 3400?cm?1, the four main Raman peaks (G, 2D, T + D and 2D) show temperature-dependent behaviour, but have different frequency shifts with increase in temperature. We propose that the peak frequency shift is related mainly to the elongation of C?C bond due to thermal expansion or anharmonic coupling of phonon modes, and oxygen-induced strong hole doping on the graphene surface. The doping effect can be confirmed from the frequency shifts, full-width at half-maximum as well as the area and intensity ratios of G and 2D peaks in temperature-dependent Raman scattering of graphene, room-temperature Raman spectra of pristine graphene and graphene cooled down after Raman measurement at 473?K in air. Therefore, the oxygen doping effect and temperature effect coexist in temperature-dependent Raman scattering of monolayer graphene.

Determination of the critical interspacing for the noninteracting magnetic nanoparticle system

The dipole–dipole interactions of monodisperse Fe3O4 nanoparticles (NPs) can be directly controlled by a uniform SiO2 shell with different thickness, i.e., different interspacings. Thus, the interacting strength of a serial of Fe3O4–SiO2 NPs system can be revealed by fitting the blocking temperature TB measured at ac fields to the Vogel–Fulcher law. The interspacing over five times of diameter for less than 8.0 nm Fe3O4 NPs is the critical value to achieve a noninteracting system. Furthermore, a general equation to evaluate critical interspacing for noninteracting magnetic NPs systems with different sizes and saturation magnetizations was calculated by Monte Carlo method.

Experimental observation of radial breathing-like mode of graphene nanoribbons

We report that single-walled carbon nanotubes (SWNTs) can be etched into graphene nanoribbons (GNRs) by iron etching, which is confirmed by Raman spectroscopy and transmission electron microscopy. Compared with SWNTs, there are some unique features in Raman spectra of GNRs: symmetric G peak with no splitting, larger Raman intensity of 2D peak than G peak, and lower frequency and narrower full width at half maximum for 2D peak. Similar to radial breathing modes in SWNTs, theoretically predicted radial breathing-like mode of GNRs is also observed: a clear and prominent peak around 223u2009cm−1 in the low frequency regions. This work paves the way for future studies of nanodevices based on SWNT-GNR heterojunction.

Controlled Fabrication of Intermolecular Junctions of Single‐Walled Carbon Nanotube/Graphene Nanoribbon

Intramolecular junctions can be formed in single-walled carbon nanotubes (SWNTs) by introducing a pentagon and/or heptagon into the hexagonal carbon lattice. The realization of these carbon-based molecular electronics is still quite challenging. Here, it is reported that nickel or cobalt catalyzed etching can be applied to partially unzip an SWNT into an intermolecular junction of SWNT/graphene nanoribbon, directly confirmed by atomic force microscopy and Raman spectroscopy.

Direct growth of graphene on gallium nitride by using chemical vapor deposition without extra catalyst

Graphene on gallium nitride (GaN) will be quite useful when the graphene is used as transparent electrodes to improve the performance of gallium nitride devices. In this work, we report the direct synthesis of graphene on GaN without an extra catalyst by chemical vapor deposition. Raman spectra indicate that the graphene films are uniform and about 5–6 layers in thickness. Meanwhile, the effects of growth temperatures on the growth of graphene films are systematically studied, of which 950 °C is found to be the optimum growth temperature. The sheet resistance of the grown graphene is 41.1 Ω/square, which is close to the lowest sheet resistance of transferred graphene reported. The mechanism of graphene growth on GaN is proposed and discussed in detail. XRD spectra and photoluminescence spectra indicate that the quality of GaN epi-layers will not be affected after the growth of graphene.

Water–Ice Transition at 274.1 K in the Channels between Single-Walled Carbon Nanotubes

Abstract The effects of confinement in channels between single-walled carbon nanotubes (SWNTs) on the water to ice transition temperature are studied in this work. The channels are provided in the SWNT rope, which are formed between SWNTs when SWNT films are treated with diamond wire drawing dies. Gold wires and aluminum tapes are used as electrodes to connect SWNT rope. When we carried out the measurement, a part of SWNT rope was dipped into water, and the electrical property of the SWNT rope was monitored with decreasing temperature. A sharp jump in voltage is found when the temperature is 274.1 K, which represents the transition temperature of confined water in the channels between SWNTs. These results indicate that the temperature of water–ice transition increases due to its confinement in nanometer size channels compared to that of bulk water. The mechanisms of our observation are discussed.