Thermal and photochemical reactivity of oxygen atoms on gold nanocluster surfaces

Reduction of oxidized gold nanoclusters by exposures to foreign gases and irradiation of UV photons has been investigated using X-ray photoelectron spectroscopy. Gold nanoclusters with narrow size distributions protected by alkanethiolate ligands were deposited on a TiO₂(1 1 0) surface with dip coating. Oxygen plasma etching was used for removal of alkanethiolate ligands and oxidization of gold clusters. The oxidized gold clusters were exposed to CO, C₂H₂, C₂H₄, H₂, and hydrogen atoms. Although, C₂H₄ and H₂ did not show any indications of reduction of oxidized gold clusters, CO, C₂H₂, and hydrogen atoms reduced the oxides on gold cluster surfaces. Among them, hydrogen atoms were most effective for reduction. Irradiation of UV photons around 400 nm could also reduce the oxidized gold clusters. The photochemical reduction mechanism was proposed as follows. The photo-reduction was initiated by electronic excitation of gold clusters and oxygen atoms activated reacted with carbon atoms at the surfaces of gold clusters. Carbon species were likely absorbed in gold clusters or remained at the boundaries between gold clusters when gold clusters agglomerated during oxygen plasma exposures. As the photochemical reduction progressed, carbon atoms segregated to the surfaces of gold clusters.     

Angle-resolved photoelectron spectroscopy of geometrically nonuniform surfaces

The formation of angle-resolved photoelectron spectra for crystal surfaces with a complex geometric relief is considered. A simple model of a thin corrugated single-crystal film is proposed which illustrates the main specific features of this process. It is shown that photoelectron spectra of these surfaces exhibit features that, rather than reflecting the true electronic structure of the system, result from its superposition with the geometric structure. The validity of the model is demonstrated with the use of a physical system based on Ni(771) and Ni(755) stepped surfaces with periodic steps. It is revealed that, as a graphite coating grows to a monolayer thickness, these surfaces become faceted to form a geometrically nonuniform surface relief. The photoelectron spectra obtained for such a surface can be used to derive a quantitative characteristic of both the electronic structure of the surface and its geometric properties. The surface topography of the system is determined independently using scanning tunneling microscopy.     

Growth and characterization of Au,Ni and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh

The nucleation and thermal stability of Au, Ni, and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh as well as the interaction between Au–Ni bimetallic clusters and reactive gases have been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Both Au and Ni atoms grow as three-dimensional (3D) clusters. Annealing the Au/carbon nanomesh surface up to 1150 °C leads to complete desorption of the Au clusters, while interfacial reaction occurs between Ni clusters and the substrate surface when the Ni clusters are subjected to the same annealing process. The nucleation of Au–Ni clusters depends critically on the deposition sequence. Au atoms preferentially nucleate on the existing Ni clusters, leading to the formation of bimetallic clusters with Au enriched on the surface. If the deposition sequence is reversed, a part of Ni atoms nucleate between the Au clusters. The thermal stability of the Au–Ni clusters resembles that of the Ni/carbon nanomesh surface, irrespective of the deposition sequence. XPS characterization reveals that Ni atoms in Au–Ni bimetallic clusters are oxidized upon exposure to 5.0 × 10^? 7 mbar O₂ for 5 min at room temperature while negligible structure change can be detected when the bimetallic clusters are exposed to CO gas under the similar conditions.     

Correlations between microstructure of plasma-modified gold nanoclusters and their optical properties

Gold nanoclusters with diameters up to 50 nm were grown in sandwich structures consisting in 15 nm of plasma deposited silicon nitride, 1 nm of gold grown by sputtering and 15 nm of plasma deposited silicon nitride (SiN/Au/SiN). Previous to the last step, ammonia plasma treatments of the gold surface were carried out with time as the main variable. The resulting structures were analyzed by high resolution transmission electron microscopy and spectroscopic ellipsometry. As a result of plasma treatments, island-like structures of as-grown gold clusters evolve to near spherical-shape features with decreasing diameter as the plasma treatment time rises. Ellipsometric spectra were modeled based on the Bruggeman effective medium approximation and the influence of size and shape of nanoparticles on the optical properties were calculated.     

Optical properties of sol-gel fabricated Ni/SiO2 glass nanocomposites

Nickel nanoparticles were grown in silica glass by annealing of the sol-gel prepared silicate matrices doped with nickel nitrate. TEM characterization of Ni/SiO₂ glass proves the formation of isolated spherical nickel nanoparticles with mean sizes 6.7 and 20 nm depending on annealing conditions. The absorption and photoluminescence spectra of Ni/SiO₂ glasses were measured. In the absorption spectra, we observed the band related to the surface plasmon resonance (SPR) in Ni nanoparticles. The broadening of SPR was observed with decrease of Ni nanoparticle size. The width of the surface plasmon band decreases 1.5 times at the lowering of temperature from 293 to 2 K because of strong electron-phonon interaction. The spectra proved the creation of nickel oxide NiO clusters and Ni²⁺ ions in silica glass as well.     

Oxidation and reduction of thin Ru films by gas plasma

The oxidation and reduction of Ru thin films grown on a Si(1 0 0) surface were studied by X-ray photoemission spectroscopy (XPS). Ru thin films were oxidized with O₂ plasma generated by an rf discharge, and their XPS spectra were measured. The spectra were decomposed into several components for Ru suboxides attributable to different stages of oxidation. After sufficient exposure to oxygen, a stoichiometric rutile RuO₂ layer was found to have formed near the surface. Thermal annealing at 500 K resulted in a thicker RuO₂ layer. Experiments demonstrated that the Ru oxide layer can be removed by H(D) atoms via the desorption of water molecules.     

LEED and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 and C) chemisorbed on the stepped rhodium (755) and (331) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₂, C₂H₄ and C has been studied on the clean stepped Rh(755) and (331) surfaces. Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) were used to determine the size and orientation of the unit cells, desorption temperatures and decomposition characteristics for each adsorbate. All of the molecules studied readily chemisorbed on both stepped surfaces and several ordered surface structures were observed. The LEED patterns seen on the (755) surface were due to the formation of surface structures on the (111) terraces, while on the (331) surface the step periodicity played an important role in the determination of the unit cells of the observed structures. When heated in O₂ or C₂H₄ the (331) surface was more stable than the (755) surface which readily formed (111) and (100) facets. In the CO and CO₂ TDS spectra a peak due to dissociated CO was observed on both surfaces. NO adsorption was dissociative at low exposures and associative at high exposures. C₂H₄ and C₂H₂ had similar adsorption and desorption properties and it is likely that the same adsorbed species was formed by both molecules.     

Tunneling spectroscopy of isolated gold clusters grown on thiol/dithiol mixed self-assembled monolayers

We report a scanning tunneling spectroscopy study on the size-tunable isolated gold nanoclusters grown on thiol/dithiol mixed self-assembled monolayers (SAMs) where the effect of neighboring clusters are practically excluded. The structure forms double tunnel junction system in which the spectra exhibit Coulomb staircases. With increasing cluster size the standard deviation of the offset charge distribution for clusters increases, accompanied with the increase of total capacitance. The results are qualitatively same with the previous ones where clusters are densely grown on the substrate, indicating that this behavior is an intrinsic property for the double tunnel junction structures of tip/vacuum/single cluster/SAMs/Au(1 1 1) systems.     

Polymer stabilized Ni-Ag and Ni-Fe alloy nanoclusters: Structural and magnetic properties

We report here the structural and magnetic behaviors of nickel-silver (Ni-Ag) and nickel-iron (Ni-Fe) nanoclusters stabilized with polymer (polypyrrole). High resolution transmission electron microscopy (HRTEM) indicates Ni-Ag nanoclusters to stabilize in core-shell configuration while that of Ni-Fe nanoclusters in a mixed type of geometry. Structural characterizations by X-ray diffraction (XRD) reveal the possibility of alloying in such bimetallic nanoclusters to some extent even at temperatures much lower than that of bulk alloying. Electron paramagnetic resonance (EPR) spectra clearly reveal two different absorption behaviors: one is ascribed to non-isolated Ni²⁺ clusters surrounded by either silver or iron giving rise to a broad signal, other (very narrow signal) being due to the isolated superparamagnetic Ni²⁺ clusters or bimetallic alloy nanoclusters. Results obtained for Ni-Ag and Ni-Fe nanoclusters have been further compared with the behavior exhibited by pure Ni nanoclusters in polypyrrole host. Temperature dependent studies (at 300 and 77 K) of EPR parameters, e.g. linewidth, g-value, line shape and signal intensity indicating the significant influence of surrounding paramagnetic silver or ferromagnetic iron within polymer host on the EPR spectra have been presented.     

Low energy alkali ion scattering investigation of Au nanoclusters grown on silicon oxide surfaces

The atomic and electronic structures of Au nanostructures grown by deposition onto various silicon oxide surfaces were probed with low energy alkali ion scattering. Charge state-resolved time-of-flight spectra of scattered 2 keV ³⁹K⁺ ions were collected from Au deposited onto an untreated Si wafer with a native oxide, a thermally grown oxide surface, and atomically-clean Si(111). It is shown that nanoclusters form on both oxides, but not on the clean Si. A quantitative analysis of the ion scattering spectra indicates that the nanoclusters are initially flat, two-dimensional structures that start to develop a second layer at about 0.5 Å of deposited Au and then form three-dimensional islands. The neutral fraction of scattered 2 keV ³⁹K⁺ ions decreases with deposition indicating changes in the quantum state occupancy with cluster size. The shapes of the clusters differ on the native and thermal oxides, leading to shape-dependent neutralization.     

大尺寸硫醇配体保护金团簇的激发态动力学研究

纳米金团簇作为桥梁连接了金纳米粒子和单个金原子,对于揭示表面等离子激元共振和金属键的来源具有重要意义。有机配体保护的纳米金团簇为理解金纳米粒子从金属性质到非金属性质的转变提供了理想的研究对象,而处于转变区域的金团簇的激发态动力学还尚待研究。在本文中,我们总结了表现出分子性质,并且尺寸较大（大于100 个金原子）的纳米金团簇的激发态动力学,同时将其与表现出金属性质的金纳米粒子的激发态动力学进行比较。本文通过对处于转变区域的金团簇的电子和振动动力学的描述,进一步讨论了其电子结构。对大尺寸纳米金团簇激发态弛豫机理的深入理解,将有助于理解金属纳米团簇和纳米粒子的光学性质,从而进一步推动这一类功能材料的设计和应用。     

Structural stability of stepped nickel surfaces

The structural stability of stepped Ni(755) and Ni(771) surfaces in the temperature range 20–500°C and during adsorption of oxygen and different forms of carbon has been studied using scanning tunneling microscopy and low-energy electron diffraction. A phase transition from the structure with double steps at room temperature to the structure with single steps at a temperature above 350°C has been observed on the clean Ni(755) surface. This transition disappears after oxygen adsorption at a temperature above 350dgC. In this case, the structure on the Ni(755) surface with single steps is stabilized in contrast to Ni(771), which tends to faceting during oxygen adsorption. It has been shown that fullerenes C60 form an array of one-dimensional chains at the upper boundaries of steps of the substrate, when they are adsorbed on the Ni(755) surface.     

高定向石墨表面金纳米粒子和金纳米线的研究

利用真空沉积方法在高定向石墨(HOPG)基底上直接制备了粒径分布较小的金纳米粒子.超高真空扫描隧道显微镜(STM)研究发现,在74℃退火后,表观直径为2.5?nm的金纳米粒子在HOPG基底上形成了排列均匀的准一维纳米粒子链,并且此金纳米粒子链结构稳定.在122℃退火后,不同粒径的金纳米粒子在HOPG基底表面上聚合长大形成了准一维金纳米线.这一发现为制备由金粒子组成的有序纳米结构开辟了探索途径.     

Structural studies of nanoporous carbon produced from silicon carbide

X-ray diffraction and small-angle scattering study of nanoporous carbon samples prepared from polycrystalline α SiC and single-crystal 6H SiC is reported. The distribution function of carbon nanoclusters in size was found. In α SiC samples, the small size (10–12 Å) of nanoclusters is combined with their high size uniformity. Graphite-like nanoclusters 30–60 Å in size were found in samples of both types. In 6H SiC samples, such clusters make up a notable fraction of the volume. The experimentally observed structural anisotropy of the samples is discussed.     

Scanning tunneling microscopy study of growth of Pt nanoclusters on thin film Al2O3/NiAl(1 0 0)

The growth of Pt nanoclusters on thin film Al₂O₃ grown on NiAl(1 0 0) was studied by using scanning tunneling microscopy (STM). The samples were prepared by vapor depositing various amounts of Pt onto the Al₂O₃/NiAl(1 0 0) at different substrate temperatures in ultra high vacuum (UHV). The STM images show that sizeable Pt nanoclusters grow solely on crystalline Al₂O₃ surface. These Pt clusters appear to be randomly distributed and only a few form evident alignment patterns, contrasting with Co clusters that are highly aligned on the crystalline Al₂O₃. The size distributions of these Pt clusters are rather broader than those of the Co clusters on the same surface and the sizes are evidently smaller. With increasing coverage or deposition temperature, the number of larger clusters is enhanced, while the size of the majority number of the clusters remains around the same (0.4 nm as height and 2.25 nm as diameter), which differs drastically from the Pt clusters on γ-Al₂O₃/NiAl(1 1 0) observed earlier. These Pt cluster growth features are mostly attributed to smaller diffusion length and ease to form stable nucleus, arising from strong Pt-Pt and Pt-oxide interactions and the peculiar protrusion structures on the ordered Al₂O₃/NiAl(1 0 0). The thermal stability of Pt nanoclusters was also examined. The cluster density decreased monotonically with annealing temperature up to 1000 K at the expense of smaller clusters but coalescence is not observed.     

Size dependent 2p3/2 binding-energy shift of Ni nanoclusters on SiO2 support: Skin-depth local strain and quantum trapping

An in situ X-ray photoelectron emission investigation revealed that the size trend of the 2p_3/2 binding-energy shift (BES) of Ni nanoclusters grown on SiO₂ substrate follows the prediction of the bond order-length-strength (BOLS) correlation theory [30]. Theoretical reproduction of the measurements turns out that the 2p_3/2 binding energy of an isolated Ni atom is 850.51 eV and its intrinsic bulk shift is 2.70 eV. Findings confirmed that the skin-depth local strain and potential well quantum trapping induced by the shorter and stronger bonds between under-coordinated surface atoms provide perturbation to the Hamiltonian and hence dominate the size dependent BES.     

Structural and electronic properties of PtPd and PtNi nanoalloys

The lowest-energy structures of binary (PtPd)_n, (PtNi)_m, (PtNi₃)_s, and (Pt₃Ni)_s nanoclusters, with n=2–28, m=2–20, and s=4–6, modeled by the many-body Gupta potential, were obtained by using a genetic-symbiotic algorithm. These structures were further relaxed within the density functional theory framework in order to obtain the most stable structures for each composition. Segregation is confirmed in all the (PtPd)_n clusters, where the Pt atoms occupy the cluster core and the Pd atoms are situated on the cluster surface. In contrast, for the (PtNi)_m nanoalloys, the Ni atoms are mainly found in the cluster core and the Pt atoms are segregated to the cluster surface. Likewise, for the (PtNi₃)_s nanoalloys, Ni atoms mainly compose the cluster core but there is no clear segregation of the Pt atoms to the surface. Furthermore, for the (Pt₃Ni)_s bimetallic clusters the Pt atoms concentrate in the cluster core and the Ni atoms are segregated to the surface. On the other hand, it has been experimentally found that the Pt_0.75Ni_0.25 supported nanoparticles present a higher catalytic activity for the selective oxidation of CO in the presence of hydrogen than the Pt_0.5Ni_0.5 and Pt_0.25Ni_0.75 nanoparticles. In order to understand this tendency in the catalytic activity, we also performed density functional calculations of the molecular CO adsorption on bimetallic Pt-Ni nanoclusters with the mentioned compositions.     

Ag-Cu离子注入玻璃后不同气氛退火的光吸收研究

采用MEVVA源（metal vapor vacuum arc ion source）引出的强束流脉冲Ag,Cu离子先后注入到SiO₂玻璃，x射线光电子能谱仪（XPS）分析显示Ag，Cu大多仍为金属态，有部分氧化态Cu存在.透射电镜观察分析和光学吸收谱都表明在衬底中形成了纳米合金颗粒.结合有效媒质理论，得到模拟的光学吸收谱，与实验结果基本符合，较好地验证了以上结论.样品退火后颗粒发生分解，分解的颗粒在氧化气氛下被氧化，且有部分向样品表面蒸发；在还原气氛下氧化态元素被还原并成核生长.故 关键词： 离子注入 纳米颗粒 退火 光学吸收率     

Adsorption of gold on oxidized tungsten

The kinetics of adsorption and desorption of gold atoms from the surface of a thin (<2 nm) oxide film grown on a textured W ribbon with the preferred emergence of the (100) face is studied using termal desorption spectrometry in a wide range of coatings. A single desorption phase is observed in the spectra of termal desorption of Au atoms from oxidized W. The activation energy of desorption of Au atoms from tungsten oxides is lower than the gold sublimation heat (it amounts to E = 3.1 eV for the concentration of adsorbate atoms on the surface corresponding to coverage θ _s = 2.38). The gold film on oxidized tungsten at room temperature grows in the form of 3D islands. The sticking coefficient for gold atoms at T = 300 K is close to unity in the coverage range 0.5 < θ _s < 2.5.     

Plasmonics: influence of the intermediate (or stick) layer on the efficiency of sensors

This paper reports the formation of Ge nanoclusters in a multi-layer structure consisting of alternating thin films of Ge-doped silica glass and SiGe, deposited by plasma-enhanced chemical vapor deposition (PECVD) and post annealed at 1100 °C in N₂ atmosphere. We studied the annealed samples by transmission electron microscopy (TEM) and Raman spectroscopy. As-deposited and annealed samples were analyzed by secondary ion mass spectroscopy (SIMS). TEM investigation shows that Ge nanoclusters were formed in the as-deposited SiGe layer and the SiGe layer was transformed into a silicon dioxide layer embedded with Ge nanoclusters after annealing. These nanoclusters are crystalline and varied in size. There were no clusters in the Ge-doped glass layer. Raman spectra verified the existence of crystalline Ge clusters. The positional shift of the Ge vibrational peak with the change of the focus depth indicates that the distribution of the stress applied to the Ge clusters varies with depth. SIMS measurements show clearly the dramatic O increase in the as-deposited SiGe layer after annealing. The creation of Ge nanoclusters by the combination of PECVD and annealing makes possible the application in complicated waveguide components. PACS 81.07.Bc; 78.66.Jg; 42.65.Wi