Showing papers by "Hong-Jun Gao published in 2008"
TL;DR: In this article, large-scale hydrophilic Fe 3O 4 nanoparticles (NPs) were prepared in the presence of citrate and sodium nitrate via a facile method.
Abstract: Large-scale hydrophilic Fe 3O 4 nanoparticles (NPs) were prepared in the presence of citrate and sodium nitrate via a facile method. The Fe 3O 4 NPs are quite stable and can be freely dispersed in water. The as-prepared magnetic nanoparticle solution can be stable for more than 1 month. The mean diameter of the Fe 3O 4 NPs can be controlled in the range of ∼20 to ∼40 nm in mean diameter. The NPs show superparamagnetic properties with a relatively high saturation magnetization moment 58 emu/mg at room temperature. Furthermore, a possible formation mechanism is proposed to explain why the magnetic nanoparticles are very well soluble in water.
275 citations
TL;DR: In this paper, Oleylamine serves as both a reducing agent and a surfactant in the synthesis of monodisperse Au, Ag, and Au3Pd nanoparticles.
Abstract: Monodisperse Au, Ag, and Au3Pd nanoparticles (NPs) with narrow size distribution are prepared by direct reaction of the related metal salt with oleylamine in toluene. Oleylamine serves as both a reducing agent and a surfactant in the synthesis. The sizes and shape of these NPs are tuned by reaction temperatures. The hydrophobic oleylamine-coated NPs can be made water soluble by replacing oleylamine with 3-mercaptopropionic acid. Both surface plasmonic resonance (SPR) and surface enhanced Raman scattering (SERS) observed from the Au and Ag NPs are found to be NP size- and surface-dependent.
183 citations
TL;DR: Experiments and first-principles calculations reveal that gold adatoms at the surface as the stable contact of the molecule to the surface is introduced as the fixed rotation axis for single tetra-tert-butyl zinc phthalocyanine molecules on an Au(111) surface.
Abstract: Molecular rotors with a fixed off-center rotation axis have been observed for single tetra-tert-butyl zinc phthalocyanine molecules on an Au(111) surface by a scanning tunneling microscope at LN2 temperature. Experiments and first-principles calculations reveal that we introduce gold adatoms at the surface as the stable contact of the molecule to the surface. An off-center rotation axis is formed by a chemical bonding between a nitrogen atom of the molecule and a gold adatom at the surface, which gives them a well-defined contact while the molecules can have rotation-favorable configurations. Furthermore, these single-molecule rotors self-assemble into large scale ordered arrays on Au(111) surfaces. A fixed rotation axis off center is an important step towards the eventual fabrication of molecular motors or generators.
120 citations
TL;DR: In this article, an array of boron nanowires (ca. 5 mu m long) vertically aligned in high density on a Si substrate is shown. But they are not shown in this paper.
Abstract: Aligned single crystalline boron nanowire arrays are formed by thermal carbon reduction. The SEM image shows an array of boron nanowires (ca. 5 mu m long) vertically aligned in high density on a Si substrate. These nanowires have a high enhancement factor, good emission stability, and can endure high current, which suggest they are an excellent candidate for field-emission applications.
96 citations
TL;DR: Submonolayer coverages of the molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) deposited on NaCl(001) surfaces were imaged with high resolution noncontact-atomic force microscopy to show the p3x3 epitaxy to be incompatible with a multilayer crystal of PTCDA.
Abstract: The nanofibrous film of polyurethane (PU) with aligned topography was fabricated by electrospinning for human umbilical vein endothelial cells(HUVEC) growth. The morphology of nanofibrous film was observed and characterized by scanning electron microscopy(SEM). The cells growth behavior including proliferation, cytoskeleton formation of actin, tublin and vinculin, and tissue factor(TF) release was investigated via cell viability assay, confocal observation and TF assay. The average diameter of the generated fiber was around 300-500 nm. The experimental results indicate that the aligned nanofibrous film of PU exhibited promotional influence on the cell proliferation. It was also observed that the film possessed an advantage of supporting HUVEC migrating and aggregating along the axis of the aligned nanofibers, which is one of the important functions in the process of endothelium regeneration. It was also demonstrated that the endothelial cells growing on the film expressed non-thrombogenic phenotype with low tissue factor released. These results indicate the favorable interactions between ECs and the film, implying that the aligned nanofibrous film of PU has a promising potential for vascular engineering.
89 citations
TL;DR: In this paper, a self-assembled localized microscopic electrochemical cell model and a diffusion-limited aggregation mode were used to explain the growth of silver dendritic nanostructures.
86 citations
TL;DR: It is shown that island nucleation is directed to the preferred sites by a much lower energy barrier and smaller critical size, while large surface energy anisotropy favors nucleation at both places.
Abstract: We develop a theoretical model to elucidate the nucleation of strained islands on patterned substrates. We show that island nucleation is directed to the preferred sites by a much lower energy barrier and smaller critical size. Strain relaxation directs island nucleation to the bottom of a pit rather than the top of a ridge as commonly perceived, while large surface energy anisotropy favors nucleation at both places. The theory explains some puzzling experimental results and provides useful guidelines for future experiments.
75 citations
TL;DR: In this article, photoresponsive azobenzene molecules were covalently grafted onto hydrogen-terminated Si(111) surfaces via Si-C linkages.
Abstract: Controlled modulation of silicon surface properties is of great importance for the development of silicon-based molecular electronic devices because of the ubiquitous role of silicon in microelectronics. In this article, photoresponsive azobenzene molecules were covalently grafted onto hydrogen-terminated Si(111) surfaces via Si-C linkages. These direct Si-C bond linkages are preferred over Si-O linkages at the interfaces because of the higher stability and the better electronic continuation between Si and the alkyl chain. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The reversible photoisomerization effects of the azobenzene molecules were also studied with contact angle measurements, atomic force microscopy (AFM), and conductive atomic force microscopy (C-AFM). The measured conductivity showed a reversible switching behavior by alternate illumination with UV and visible light. Thus, we have demonstrated molecularly controlled modulation of conductance of the Si surface by the photochemical method. Furthermore, the dipole moments of the azobenzene molecules switched accordingly with the alternate illumination. Making use of this characteristic, we have provided a strategy to evaluate the influence of the molecular dipole moments on the conductance of the semiconductor surface.
47 citations
TL;DR: In this paper, flexible boron nanowires have been synthesized via thermoreduction via heating with magnesium and they have been shown to have excellent mechanical properties and resistance to mechanical fracture even under a strain of 3%.
Abstract: Flexible boron nanowires have been synthesized via thermoreduction in boron-oxygen compounds with magnesium. These as-prepared nanowires, which are structurally uniform and single crystalline, represent good semiconductor at high temperature. Tensile stress measurements demonstrate excellent mechanical property of boron nanowires as well as resistance to mechanical fracture even under a strain of 3%. Importantly, simultaneous electrical measurement reveals that the corresponding electrical conductance is very robust and remains constant under mechanical strain. Our results can be briefly explained by Mott’s variable range hopping model.
33 citations
TL;DR: A tentative assignment of the preferred adsorption sites is proposed from a comparison of the measured and computed g values and hyperfine coupling constants (hfccs), and the reasons for this different behavior of hydrogen atoms are discussed.
Abstract: The adsorption of small amounts of alkali metal atoms (Li, Na, K, Rb, and Cs) on the surface of MgO powders and thin films has been studied by means of EPR spectroscopy and DFT calculations. From a comparison of the measured and computed g values and hyperfine coupling constants (hfccs), a tentative assignment of the preferred adsorption sites is proposed. All atoms bind preferentially to surface oxide anions, but the location of these anions differs as a function of the deposition temperature and alkali metal. Lithium forms relatively strong bonds with MgO and can be stabilized at low temperatures on terrace sites. Potassium interacts very weakly with MgO and is stabilized only at specific sites, such as at reverse corners where it can interact simultaneously with three surface oxygen atoms (rubidium and cesium presumably behave in the same way). Sodium forms bonds of intermediate strength and could, in principle, populate more than a single site when deposited at room temperature. In all cases, large deviations of the hfccs from the gas-phase values are observed. These reductions in the hfccs are due to polarization effects and are not connected to ionization of the alkali metal, which would lead to the formation of an adsorbed cation and a trapped electron. In this respect, hydrogen atoms behave completely differently. Under similar conditions, they form (H(+))(e(-)) pairs. The reasons for this different behavior are discussed.
30 citations
TL;DR: In this article, the interaction of Li atoms with terrace sites of MgO(0 0 1) surfaces is characterized by combining EPR spectroscopy with theoretical calculations.
TL;DR: Using first-principles density functional theory calculations, this article revealed that the nature of the PTCDA and Ag(111) interface is characterized by multichannel molecular orbital interactions.
Abstract: Using first-principles density functional theory calculations, we reveal that the nature of the PTCDA and Ag(111) interface is characterized by multichannel molecular orbital interactions. The interacting channels via the occupied electronic states are primarily located at the periphery of PTCDA, whereas those via the unoccupied states are at the center of PTCDA. Our theory provides a unified picture explaining all the exciting experiments. Also, a confined two-dimensional free-electron-like interface state is discussed.
TL;DR: In this article, the behavior of various Ge nanostructures fabricated on Si(111)-7 × 7, ranging from the initial adsorption sites of individual Ge atoms to the aggregation patterns of Ge nanoclusters, and then to 2D extended Ge islands, are comprehensively investigated by STM.
Abstract: Firstly, both the rest atoms and the adatoms of Si(111)-7×7 surface are observed simultaneously by scanning tunneling microscopy (STM) when the sample bias voltages are kept less than -0.7V. The visibility of the rest atoms is rationalized by first-principle calculations and a very sharper tip can resolve them. Secondly, the behaviors of various Ge nanostructures fabricated on Si(111)-7 × 7, ranging from the initial adsorption sites of individual Ge atoms to the aggregation patterns of Ge nanoclusters, and then to 2D extended Ge islands, are comprehensively investigated by STM. The individual Ge atoms tend to substitute for Si adatoms at Si(111)-7 × 7 with the preference of corner adatoms in the faulted half unit when keeping substrate at 150°C. With increasing Ge coverage, individual Ge atoms and Ge nanoclusters coexist on the substrate. Subsequently, the density of Ge nanoclusters increase and cluster-distribution becomes gradually regular with the formation of final 2D extended hexagonal configuration. When keeping the substrate at 300°C, Ge islands consisting of more complicated reconstructions with intermixing Ge/Si components are present on the substrate. The detail structural characterizations and the bonding nature of the observed Ge nanostructures are enunciated by the first-principle calculations.
TL;DR: In this paper, a two-step model at different temperatures and the poor wettability between boron carbide and silicon oxide play important roles in the growth of hetero-nanonecklaces.
Abstract: Boron carbide and silicon oxide (BCSiO) hetero-nanonecklaces have been successfully synthesized via temperature modulation. This kind of nanostructure was formed by coating 100-500 nm silicon oxide nanoballs onto 20-30 nm boron carbide nanowires. Synthetic analysis shows that a two-step model at different temperatures and the poor wettability between boron carbide and silicon oxide play important roles in the growth of hetero-nanonecklaces. Photoluminescence of the synthesized BCSiO hetero-nanonecklaces shows enhanced visible light emissions at 637.6 nm, which is attributed to the small size of the boron carbide nanowires and defects induced by silicon oxide sheaths.
TL;DR: It is found that the adsorption configuration depends on the coverage, and that functionalizing the benzene ring with one or two fluorine atoms weakens the bonding to the surface.
Abstract: We modelled the adsorption of benzene, fluorobenzene and meta-di-fluorobenzene on Cu(110) by Density Functional Theory. We found that the adsorption configuration depends on the coverage. At high coverage, benzene assumes a tilted position, while at low coverage a horizontal slightly distorted geometry is favoured. Functionalizing the benzene ring with one or two fluorine atoms weakens the bonding to the Surface. A rotation is induced, which decreases the distance of the fluorine atom from the surface. STM simulations reveal that details about both, benzene adsorption geometry and fluorine position, can be only detected at short tip-surface distances. (C) Crown copyright 2008.
Posted Content•
TL;DR: In this paper, a large deviation principle for the two-layer quasi-geostrophic flow model under uncertainty is proved based on the Laplace principle and a variational approach.
Abstract: The stochastics two-layer quasi-geostrophic flow model is an intermediate system between the single-layer two dimensional barotropic flow model and the continuously stratified three dimensional baroclinic flow model. This model is widely used to investigate basic mechanisms in geophysical flows, such as baroclinic effects, the Gulf Stream and subtropical gyres.
A large deviation principle for the two-layer quasi-geostrophic flow model under uncertainty is proved. The proof is based on the Laplace principle and a variational approach. This approach does not require the exponential tightness estimates which are needed in other methods for establishing large deviation principles.
TL;DR: A dipolar Ru(II) complex was synthesized and characterized and a multilayer film of at least 18 layers was successfully prepared by alternating adsorption of H4SiW12O40 and [Ru2(bpy)4(bpbh)](ClO4)4 by electrostatic layer-by-layer self-assembly.
Abstract: A dipolar Ru(II) complex, [(bpy)2Ru(bpbh)Ru(bpy)2](ClO4)4 {where bpbh = 1,6-bis-[2-(2-pyridyl) benzimidazoyl]hexane, bpy = 2,2'-bipyridine}, was synthesized and characterized. A multilayer film of at least 18 layers was successfully prepared by alternating adsorption of H4SiW12O40 and [Ru2(bpy)4(bpbh)](ClO4)4 by electrostatic layer-by-layer self-assembly. The multilayer films were studied by ultraviolet-visible and X-ray photoelectron spectroscopy, atomic force microscopy, and cyclic voltammetry.
TL;DR: In this article, a reversible and reproducible conductance transition is realized in the monolayer of polyimide (PI) Langmuir−Blodgett film on Au (111) substrate with a scanning tunneling microscopy (STM).
Abstract: A reversible and reproducible conductance transition is realized in the monolayer of polyimide (PI) Langmuir−Blodgett film on Au (111) substrate with a scanning tunneling microscopy (STM). The high-conductance state can be induced by applying a voltage pulse of about −2.7 V using the STM lithography method. The PI monolayer can spontaneously recover to its original low conductance state under a normal STM scanning condition. An electron trapping/detrapping process in the PI molecule can be responsible for its conductance transition. The reversible and highly reproducible conductance switching behavior in the PI film could provide its potential applications in the molecular logic and memory devices.
TL;DR: In this paper, the growth behavior of a layer of a quinacridone derivative (QA16C) on the Ag(110) surface was studied using low-temperature scanning tunneling microscopy.
Abstract: The growth behavior of a layer of a quinacridone derivative (QA16C) on the Ag(110) surface was studied using low-temperature scanning tunneling microscopy. At very low coverage, molecule−substrate interactions determine the adsorption sites of QA16C molecules on Ag(110), and two distinct orientations of single molecules can be observed on silver terraces. At higher coverages up to 1 ML, intermolecular interactions drive the QA16C molecules to form rowlike nanostructures. Meanwhile, step-edge alignments of silver substrate induced by adsorbed organic molecules can be observed at very low coverage and at monolayer coverage. When the QA16C molecule coverage increases further to the second layer, different rowlike structures are formed. Intermolecular interactions, such as hydrogen bonds and alkyl−alkyl and π−π interactions, dominate the second-layer structure. In addition, we show that the second layer of QA16C molecules can be controlled by increasing the growth temperature during deposition.
Journal Article•
TL;DR: In this article, the authors considered a mathematical model for the coupled atmosphere-ocean system, namely, the coupled quasigeostrophic-flow-energy balance model, and showed that the system is ergodic.
Abstract: The authors consider a mathematical model for the coupled atmosphere-ocean system, namely, the coupled quasigeostrophic-flow-energy-balance model. This model consists of the large-scale quasigeostrophic oceanic flow model and the transport equation for oceanic temperature, coupled with an atmospheric energy-balance model. After reformulating this coupled model as a random dynamical system (cocycle property), it is shown that the coupled quasigeostrophic-energy balance fluid system has a random attractor, and under further conditions on the physical data and the covariance of the noise, the system is ergodic, namely, for any observable of the coupled atmosphere-ocean flows, its time average approximates the statistical ensemble average, as long as the time interval is sufficiently long.