Formation of ordered arrays of gold particles by nanoindentation templating

Ordered arrays of gold‐rich particles, with diameters ranging from 50 nm to 180 nm, have been formed on a silicon (100) surface through pre‐patterning by nanoindentation. Indentation and gold deposition of the sample is followed by thermal processing, causing the gold to become trapped at the indentation sites. We suggest that gold trapping is via an alloying process with the underlying Si substrate where the native oxide is structurally compromised by the indentation process. The final size for a given particle is directly dependent on the size of the indentation site. It has also been demonstrated that excess gold found on the surface outside of these indentation sites can be readily removed via simple mechanical abrasion without affecting the particles within the indent. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation of cold bialkali dimers on helium nanodroplets

Cold alkali diatomic molecules (LiCs, NaCs) in the lowest vibrational state of the electronic triplet ground state are formed on superfluid helium nanodroplets. Using photoionization detection the excitation spectra of the transitions are recorded. The splitting of the vibrational structure in the LiCs spectrum, not observed in the NaCs spectrum, is interpreted in terms of molecular fine structure. The spectra are well reproduced by a model based on quantum chemistry potential curves including spin-orbit coupling, in combination with an asymmetric line shape function to account for cluster-induced broadening. Our refined potential curves provide important input data for the photoassociation of ultracold dipolar alkali molecules from atomic quantum gases.Received: 1 July 2004, Published online: 26 October 2004PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters - 34.50.Gb Electronic excitation and ionization of molecules; intermediate molecular states (including lifetimes, state mixing, etc.) - 33.20.-t Molecular spectra     

Formation of small particles of gold on alumina support films and their behaviour in oxygen and hydrogen atmospheres

Attempt has been successfully made to disperse ultrafine gold particles on alumina support films by keeping the substrate at elevated temperatures during metal deposition under vacuum of ≈ 10^-5 Torr. It is found that reasonable dispersion of particles occurs for a mean thickness of 0.5 nm at a substrate temperature of 125 °C. The resulting samples turn out to be quite suitable for model studies of the behaviour of gold/alumina catalyst. Electron microscope observations coupled with diffraction have provided evidence for emergence of gold particles without any compound formation with the residual gases present during deposition at 10^-5 Torr. The nature of dispersion and average particle size are shown to depend on substrate temperature and the amount of metal evaporated. Heat treatments in hydrogen and oxygen at 200–500 °C for various lengths of time led to an increase in particle size with a simultaneous decrease in number density (i.e. coarsening of particles) following, in all probability, the Ostwald ripening mechanism. The oxidizing atmosphere has been shown to be more favourable to coarsening or deactivation of the catalyst. Finally, it is indicated that gold particles prefer to facet giving well-defined shapes (e.g. hexagonal, pentagonal or rhombohedral) with faces corresponding to planes of relatively smaller interfacial energies.     

Fano resonances in heterogeneous dimers of silicon and gold nanospheres

We theoretically investigate the optical properties of dimers consisting of a gold nanosphere and a silicon nanosphere. The absorption spectrum of the gold sphere in the dimer can be significantly altered and exhibits a pronounced Fano profile. Analytical Mie theory and numerical simulations show that the Fano profile is induced by constructive and destructive interference between the incident electric field and the electric field of the magnetic dipole mode of the silicon sphere in a narrow wavelength range. The effects of the silicon sphere size, distance between the two spheres, and excitation configuration on the optical responses of the dimers are studied. Our study reveals the coherent feature of the electric fields of magnetic dipole modes in dielectric nanostructures and the strong interactions of the coherent fields with other nanophotonic structures.     

劈裂环-盘二聚体结构的多重Fano共振

利用时域有限差分方法,理论研究了由劈裂环和圆盘构成的金二聚体结构的光学性质,分析了劈裂环的缺口取向和对称性破缺程度对其Fano共振特性的影响.结果表明,当缺口方向平行于二聚体中心连线时,劈裂环的奇数阶和偶数阶模式均能与圆盘的偶极模式作用产生Fano共振,且随着劈裂环的进一步破缺,更多的偶数阶Fano共振能被激发出来;但...     

Stabilizing oxide layers on iron particles

Pyrophoric iron powders at different stages of stabilization by oxygen (0–8% by weight) were annealed at 255 or 500°C up to 571 h. During this procedure the magnetic moments first decreased by some per cent and then increased above the initial values. The shape of the magnetic moment versus oxygen uptake curves remained unchanged. The increase of the magnetic moments as a function of time is correlated with a decrease of the specific surface area of the powders. The Mössbauer spectrum of partially stabilized powders is different from that observed on completely stabilized powders and shows a hitherto unknown magnetic iron oxide while X-ray investigations suggest this oxide to be a special iron deficient Fe₃O₄ (spinel) containing vacancies on the octahedral sites distributed at random. The spins in the oxide should then be coupled in an antiparallel way to the underlying iron core in order to explain the measured magnetic moments.     

Heterodyne holographic microscopy of gold particles

We report experimental results on heterodyne holographic microscopy of subwavelength-size gold particles. The apparatus uses continuous green-laser illumination of the metal beads in a total internal reflection configuration for dark-field operation. Detection of the scattered light at the illumination wavelength on a charge-coupled-device array detector enables 3D localization of brownian particles in water.     

Formation and destruction of nitric oxide in methane flames doped with NO at atmospheric pressure

A study of formation and destruction of NO in adiabatic laminar premixed flames of CH₄ + O₂ mixtures diluted with N₂ or Ar (with various dilution ratios) in a range of equivalence ratios at atmospheric pressure is presented. Nitric oxide was seeded into the flames using mixtures of diluent gas + 100 ppm of NO. The heat flux method was employed to measure adiabatic burning velocities of these flames. Nitric oxide concentrations in the post-flame zone at 10, 15 and 20 mm above the burner surface were measured using probe sampling. Burning velocities and NO concentrations simulated using a previously developed chemical kinetic mechanism were compared with the experimental results. The conversion ratio of NO seeded into the flames was determined. The kinetic mechanism accurately predicts burning velocities over the range of equivalence ratios and NO conversion in the rich flames. Significant discrepancies between measured and calculated NO conversion in the lean and near-stoichiometric flames were observed and discussed.     

NO气体的空间交叉CARS

采用具有高空间分辨率的空间交叉BOX CARS相位匹配方法,完成了对NO气休室温下、Q支(v=0→v=1)振转CARS谱的测量,研究了它与NO气体压强及激光线宽的关系.从CARS基本理论出发,考虑到激光线宽及不同线型,计算出理论曲线,与实验谱线进行了比较,洛伦兹线型的理论模拟和实验结果符合较好.     

Formation of silver particles and silver oxide plume nanocomposites upon pulsed-laser induced liquid-solid interface reaction

Nanodendrites consisting of silver and silver oxide are grown upon a pulsed-laser induced reaction at the interface between the solid target and silver nitrate solution. By using a high-resolution transmission electron microscope (HRTEM) equipped with an energy dispersive X-ray spectrometer (EDS) and selected area diffraction (SAD), the fabricated nanopatterns are identified to be a composite structure that consisting of silver nanoparticles and silver oxide nanoplumes with polycrystalline structure. In detail, these silver nanocrystals are trunks of the nanodendrite, and their size is in the range of 30 to 50 nm. The silver oxide nanoplumes are branches of the nanodendrite, and their width and length are in the ranges of 20 to 50 nm and 30 to 100 nm, respectively. Additionally, we suggested a vapor-liquid mechanism for the formation of the nanopatterns using a pulsed-laser induced liquid-solid interface reaction, in which both silver clusters in vapor and silver ions in solution are simultaneously involved.Received: 6 April 2004, Published online: 5 November 2004PACS: 81.15.Fg Laser deposition - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 81.07.Bc Nanocrystalline materials     

Physical adsorption of nitric oxide on metal oxides

Adsorption isotherms have been measured at 77.5 K for nitric oxide and nitrogen on Al₂O₃, MgO, ZnO and NiO, and at 90.2 K. for nitric oxide on A1₂O₃ and NiO. Three isotherm measurements at 77.5 K were made on the Al₂O₃ sample for each adsorbate to examine the effect of different degrees of surface dehydroxylation. The latter was assessed by means of infrared absorption studies on an Al₂O₃ disc. Isosteric heats for NO adsorption on Al₂O₃ and NiO increase from ca. 8 kJ mol^?1 and 6 kJ mol^?1 (respectively) at half monolayer coverage to near the value of the enthalpy of sublimation (16.6 kJ mol^?1) at monolayer completion. These results are discussed in terms of adsorbate dimerisation. Anomalous adsorption-desorption behaviour for the NONiO system is discussed. Effective adsorbate molecular cross-sectional areas and results for N2 adsorption on preadsorbed NO do not support the existence of either localisation or micro-porosity.     

Chemisorption and decomposition of nitric oxide on ruthenium

Nitric oxide, strongly chemisorbed on ruthenium, is desorbed almost completely as oxygen and nitrogen. Oxygen, nitrogen, and nitric oxide were observed singly on ruthenium with field emission microscopy. Thermal desorption spectroscopy from Ru(101̄0) showed that molecular nitrogen is only physisorbed but nitrogen from NO decomposition is strongly chemisorbed. Nitrogen from NO shows three binding states, the most strongly bound being present to only a small extent. NO shows three and two binding states when adsorbed at 120 K and 295 K respectively. Work function measurements gave Δφ = 1.3 eV for a monolayer of NO. NO is dissociatively adsorbed above 250 K but a lower temperature limit was not established. The decomposition of NO on $\text{Ru}\text{(10}\text{1}\text{0)}$ under high vacuum conditions is catalytic in that no oxides of ruthenium were observed to form in the process.     

Optical absorption in ultrafine gold particles

Optical absorption of ultrafine, gas evaporated gold particles (diameters 3—4 nm) has been measured in the wavelength interval 0.3—2.5 μm. The data agree well with the Maxwell-Garnett theory in conjunction with optical constants for bulk Au, provided these are modified to incorporate the effect of size dependent electron scattering.     

Absorption spectrum of surface-bound cap-shaped gold particles

A surface-adsorbed monolayer of cap-shaped gold particles upon submicrometer-sized polystyrene spheres exhibits pronounced absorption in the visible region. When the surrounding refractive index was altered by immersion in a fluid, the direction of the shift in the absorption spectrum was dependent on the incidence angle of the irradiation. When a thiol molecule, known to adsorb selectively on gold upon polystyrene, was added, the resultant shift in the absorption spectrum's peak was consistently toward longer wavelengths. Consequently, at certain incidence angles, a change in the refractive index of the surrounding fluid produces no shift, whereas thiol adsorption results in a clear shift, apparently reflecting the different spatial regions in which the refractive index is altered by these two procedures.     

Chemisorption of nitric oxide by nickel

The adsorption of nitric oxide on clean and pre-oxidized nickel has been investigated by X-ray photoelectron spectroscopy. Three distinct states of chemisorption have been recognised at room temperature; one is dissociative while two involve molecularly adsorbed NO. Pre-exposing the nickel surface to oxygen enabled the activity of the surface to be controlled such that adsorption was confined to only one of the molecular states. The two molecular states are suggested to arise from “bent” and “linear” forms of NO.     

In-situ XPS investigation of nitric oxide adsorption on (111), (310), and (533) gold single crystal faces

Decomposition of nitric oxide on the flat (111) and stepped (310) and (533) faces of gold single crystals was investigated by in-situ X-Ray Photoelectron Spectroscopy at elevated NO pressures up to 7 Pa and temperatures between 300 K and 500 K. The adsorbed layer formed on the gold surfaces at these conditions was found to contain nitrogen adsorbed atomically and/or nitrous oxide. The population of these adspecies was found to be dependent upon the crystal structure of the Au face, as well as the sample temperature and NO pressure. Explanations for these phenomena are discussed.     

Heterodyne frequency measurements on the nitric oxide fundamental band

Heterodyne frequency measurements have been made on the fundamental band of nitric oxide from 1750 to 1931 cm⁻¹. Based on the analysis of these new measurements, minor changes are made in the band constants and an improved list of calculated energy levels for the v = 0 and v = 1 states is given.     

Initial oxidation of MgO-supported Rh nanoparticles studied by TEM

The formation of ultrathin oxide layers on the facets of MgO(0 0 1) supported Rh nanoparticles is revealed by high-resolution transmission electron microscopy (HRTEM) and spatially-resolved electron energy-loss spectroscopy (EELS). An O–Rh–O trilayer surface oxide has been observed on both {1 1 1} and (0 0 1) facets, which is confirmed by image simulation using the atomic model of a two-dimensional surface oxide obtained by density functional theory (DFT). The spacing between the oxide layer and the Rh {1 1 1} facet is however markedly smaller, indicating a variation of interface bonding in the case of nanoparticles. When the oxide layer is slightly thicker with two Rh planes, the structure of the surface oxide is different from corundum Rh₂O₃ bulk oxide, and the trilayer surface oxide persists as terminating layer. The spacing between the oxide layer and the Rh(1 1 1) facet is found to vary, being smaller in the middle of the oxide layer. It is also found that oxidation is more pronounced at the intersections of the nanoparticles’ facets.