Nanospherical Surface‐Supported Seeded Growth of Au Nanowires: Investigation on a New Growth Mechanism and High‐Performance Hydrogen Peroxide Sensors

In this paper, a novel strategy with a new growth mechanism for fast and large‐scale growth of Au long nanowires on high‐curvature SiO₂ nanospherical surfaces has been developed. The synthesis includes three steps, i.e., amino modification of SiO₂ nanospheres, Au seed loading on aminated SiO₂ nanospheres and subsequently, Au seed‐mediated nanowire growth on SiO₂ nanospheres. The prepared Au nanowires (Au NWs) (exhibit long length, high aspect ratio, and good flexibility, and can naturally form the dense nanowire film, which is promising as a stable conductive electrode. In addition, the effect of synthetic conditions such as reactant feeding order, Au seeds and SiO₂@Au seeds on the morphology of Au nanostructures (nanowires, nanoteeth, and nanoflowers) has been investigated. It is found that Au seeds and high‐curvature SiO₂ nanospherical surfaces are necessary conditions for the successful preparation of Au NWs and nanowire films. The different growth mechanisms for Au NWs and nanoteeth have been proposed and discussed. Moreover, the novel nonenzymatic H₂O₂ sensor based on Au NWs exhibits much enhanced performance such as higher sensitivity, stability, and selectivity, wider linear range and lower detection limit, compared with that of Au nanoparticles‐based H₂O₂ sensor.     

Magnetoresistive properties of Fe3O4 nanoparticles embedded in a Cu matrix

We studied ordered arrays of magnetic nanoparticles (NPs) in a nonmagnetic matrix. The influence of annealing temperature and measurement geometry (varying angle between sample surface and external magnetic field direction) on magnetoresistance and coercive field values was established. Measurements were done on the Au(2 nm)/Cu(20 nm)/Fe₃O₄(NPs)/SiO₂/Si system.     

Dynamics and instability of solid-state dewetting

Spontaneous dewetting of solid thin films proceeds by edge retraction of film edges and/or by heterogeneous void growth. Classical 1D and 2D continuous models of the evolution of a dewetting film, based on surface diffusion mechanisms, predict that in the long-time limit dewetting obeys universal scaling laws. In this paper, we review 1D and 2D predictions and recent experimental results. For this purpose, using Si(001)/SiO₂ and Ge(001)/SiO₂ single-crystalline thin films in different geometries, we have been able to compare theoretical predictions to experimental results obtained by combining in situ LEEM and ex situ AFM measurements. For dewetting from film edges, experimental results partially differ from continuous models predictions. More precisely, because of the crystallographic anisotropy: (i) the facetted edges remain stable during dewetting (they simply recede at constant shape) while poorly or un-facetted edges are unstable (they recede by finger formation); (ii) rim formation, induced by mass-conservation condition, proceeds in a layer-by-layer mode and is limited by 2D nucleation properties on the top of the rim; (iii) the island generation mechanism differs from the mass shedding behaviour predicted by 1D models. For dewetting mechanisms involving void growth, different behaviours are reported and discussed. For thin Si(001)/SiO₂ films, the corners of the opening square-shaped voids lead to a local destabilisation of the growing voids. For thin Ge(001)/SiO₂ films, the side of the voids invariably turns instable and forms tip dendrites whose branch density depends on the temperature and the initial film thickness. Finally, ultra-thin films, more sensitive to local fluctuations, dewet in a fractal geometry.     

Si-W/Si/SiO2/Si(100)的横截面电子显微镜研究

本文用横截面电子显微镜法分析了Si-W/Si/SiO₂/Si(100)在440—1000℃退火后的晶化过程,以及各个界面的变化情况.发现Si-W合金膜中,WSi₂并未优先在表面、界面处形成晶核.当退火温度不高于700℃时,反应在合金膜内发生,表面、界面起伏和缓.退火温度高达800—1000℃时,界面、表面出现原子扩散,造成剧烈的界面起伏;表面则出现小的热沟槽,Si/SiO₂界面也出现高分辨电子显微镜才能观察到的起伏.表面、界面的原子迁移的动力来源于晶界与表面、界面张力.由于SiO₂中Si—O键很稳定,不易发生Si和O在界面处的互扩散,所以Si/SiO₂界面起伏很小. 关键词：     

Crystallization and surface morphology of Au/SiO2 thin films following furnace and flame annealing

A crystallization and surface evolution study of Au thin film on SiO₂ substrates following annealing at different temperatures above the eutectic point of the Au/Si system are reported. Samples were prepared by conventional evaporation of gold in a high vacuum (10⁻⁷ mbar) environment on substrates at room temperature. Thermal treatments were performed by both furnace and flame annealing techniques. Au thin films can be crystallized on SiO₂ substrates by both furnace and flame annealing. Annealing arranges the Au crystallites in the (1 1 1) plane direction and changes the morphology of the surface. Both, slow and rapid annealing result in a good background in the XRD spectra and hence clean and complete crystallization which depends more on the temperature than on the time of annealing. The epitaxial temperature for the Au/SiO₂ system decreases in the range of 350-400 °C. Furnace and flame annealing also form crystallized gold islands over the Au/SiO₂ surface. Relaxation at high temperatures of the strained Au layer, obtained after deposition, should be responsible for the initial stages of clusters formation. Gold nucleation sites may be formed at disordered points on the surface and they become islands when the temperature and time of annealing are increased. The growth rate of crystallites is highest around 360 °C. Above this temperature, the layer melts and gold diffuses from the substrate to the nucleation sites to increase the distance between islands and modify their shapes. Well above the eutectic temperature, the relaxed islands have hexagonally shaped borders. The mean crystallite diameters grow up to a maximum mean size of around 90 nm. The free activation energy for grain boundary migration above 360 °C is 0.2 eV. Therefore the type of the silicon substrate changes the mechanism of diffusion and growth of crystallites during annealing of the Au/Si system. Epitaxial Au(1 1 1) layers without formation of islands can be prepared by furnace annealing in the range of 300-310 °C and by flame annealing of a few seconds and up to 0.5 min.     

Size and density control of silicon oxide nanowires by rapid thermal annealing and their growth mechanism

In this work, we demonstrate a fast approach to grow SiO₂ nanowires by rapid thermal annealing (RTA). The material characteristics of SiO₂ nanowires are investigated by field emission scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field (HAADF) imaging, electron energy loss spectroscopy (EELS), and energy-filtered TEM (EFTEM). The HAADF images show that the wire tip is predominantly composed of Pt with brighter contrast, while the elemental mappings in EFTEM and EELS spectra reveal that the wire consists of Si and O elements. The SiO₂ nanowires are amorphous with featureless contrast in HRTEM images after RTA at 900°C. Furthermore, the nanowire length and diameter are found to be dependent on the initial Pt film thickness. It is suggested that a high SiO₂ growth rate of >1 μm/min can be achieved by RTA, showing a promising way to enable large-area fabrication of nanowires.     

Interfacial reaction during dewetting of ultrathin silicon on insulator

We have studied the Si/SiO₂ interfacial reaction during solid-state dewetting of 7-nm-thick Si(001) ultrathin films on SiO₂ substrates. Immediately after formation of Si nanocrystals at the dewetting front, Si/SiO₂ interface depression occurs at the edge of the nanocrystal because of the interfacial reaction. By examining the Si/SiO₂ interface morphology for nanocrystals at different distances from the dewetting front, we found that the interface depth increases linearly with time. We also estimated that the effective activation energy for the interfacial depression is about 3.9 eV. Furthermore, we explain the effect of the interfacial reaction on the active morphological change involved in dewetting front propagation.     

Silicon nanowires synthesized by vapor–liquid–solid growth on excimer laser annealed thin gold film

The fabrication of Si nanowires has been demonstrated using excimer laser annealed thin gold film as the catalyst and vapor–liquid–solid (VLS) growth. Au nanoparticles with mean diameters of 12, 13 and 15 nm were formed by excimer laser annealing (ELA) of Au film with thickness of 2.5, 5 and 10 nm, respectively. The results show that the silicon nanowires (SiNWs) with desired diameter can be obtained by controlling the Au film thickness and laser power density.     

Photoluminescence from amorphous silica nanowires synthesized using TiN/Ni/SiO2/Si and TiN/Ni/Si substrates

Photoluminescence characteristics of amorphous silica nanowires (a-SiONWs) grown on TiN/Ni/Si and TiN/Ni/SiO₂ substrates have been studied. A-SiONWs grown on TiN/Ni/Si substrates show a Si-rich composition compared to those grown from TiN/Ni/SiO₂/Si. The emission characteristics of the nanowires were found to depend on the type of substrate. By annealing the a-SiONWs grown on TiN/Ni/Si in air, emission bands shift from blue to green bands. It is likely that silicon to oxygen ratio is an important factor in deciding the types of defects and emission bands of amorphous silica nanowires.     

Fabrication and annealing effects of SnO2/SiOx nanocables sheathed by the sputtering technique

We reported an approach, in which we have produced the nano-sized crystalline tin oxide (SnO₂) particles with rutile structure. SnO₂ nanowires were coated with a shell layer of SiO_x via a sputtering method. Transmission electron microscopy and elemental mapping investigations revealed that the nanostructures consisted of a crystalline SnO₂ core surrounded by an amorphous SiO_x sheath. The annealing effects on the core-shell nanowires were investigated, revealing that the outer surface became rougher by the thermal annealing. For core-shell nanowires, a room-temperature PL measurement with a Gaussian fitting showed yellow, blue, and violet light emission bands, with the relative intensity of the yellow band showing an increase after thermal annealing. Possible PL emission mechanisms are discussed. This study reveals that the sputtering is effective for preparing the shell layers of nanocables.     

Surface enhanced infrared absorption on Au nanoparticle films deposited on SiO2/Si for optical biosensing: Detection of the antibody-antigen reaction

The specific antibody-antigen reaction was detected by taking advantage of the effect of surface enhanced infrared absorption (SEIRA). For the SEIRA active film, single Au nanoparticles (AuNP) deposited on the SiO₂/Si wafer surface were used. After immobilizing specific antibodies onto the AuNP, these samples were then exposed to specific antigens (unspecific antigens, respectively); then the samples were investigated with infrared spectroscopy. In addition, the same kind of sample preparation was done using a 40 nm thick non-enhancing Au film, in order to compare the SEIRA transmission spectra to the infrared reflection absorption spectra (IRRAS) and therefore to determine the factor of the SEIRA enhancement. In both geometries, SEIRA transmission and IRRAS, the antibody-antigen coupling could clearly be detected; a comparison of the spectra showed, that the enhancement factor due to the Au nanoparticles is roughly 25.     

Formation of silicon oxide nanowires directly from Au/Si and Pd–Au/Si substrates

Amorphous silicon oxide (SiO_x) nanowires were directly grown by thermal processing of Si substrates. Au and Pd–Au thin films with thicknesses of 3 nm deposited on Si (0 0 1) substrates were used as catalysts for the growth of nanowires. High-yield synthesis of SiO_x nanowires was achieved by a simple heating process (1000–1150 °C) in an Ar ambient atmosphere without introducing any additional Si source materials. The as-synthesized products were characterized by field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy measurements. The SiO_x nanowires with lengths of a few and tens of micrometers had an amorphous crystal structure. The solid–liquid–solid model of nanowire formation was shown to be valid.     

Preferred orientations in nano-gold/silica/silicon interfaces

Gold in contact with silicon substrates Si(1 0 0), Si(1 1 1), and SiO₂ is studied by thermal evaporation and annealing in N₂ using the modified sphere-plate technique. The final orientation distribution of crystalline Au films grown on Si substrate systems that incorporate a native amorphous oxide layer of silica and Au on amorphous silica (SiO₂ glass) substrates is influenced by preferred orientations and twinning. Experimental evidence suggests that the orientation of Au{1 1 1} close packed planes (multiply twinned) was found to be of low-energy as the annealing temperature was increased to 530 °C and 920 °C. Additional orientations were observed for Au{1 0 0} on Si(1 0 0) substrates and Au{1 0 0}, {1 1 0}, and {3 1 1} on SiO₂ substrates. After annealing at 920 °C the size distribution of the gold particles was determined to be within the range of 20-800 nm while the morphology of gold surface appears spherical to faceted in character. These results show similarities to recent findings for smaller nano-size 1D particles, islands and thin Au films on silicon annealed over lower temperature ranges.     

Formation of epitaxial SiC nanocrystals

Epitaxial 3C-SiC grains are formed at 1190 °C in the top region of silicon, when Si wafers coated by SiO₂ are annealed in CO atmosphere. The formed SiC grains are 40-50 nm high and 100 nm wide in cross-section and contain only few defects. Main advantage of the method is that the final structure is free of voids.The above method is further developed for the generation of SiC nanocrystals, embedded in SiO₂ on Si, and aligned parallel with the interface. The nanometer-sized SiC grains were grown into SiO₂ close to the Si/SiO₂ interface by a two-step annealing of oxide covered Si: first in a CO, than in a pure O₂ atmosphere. The first (carbonization) step created epitaxial SiC crystallites grown into the Si surface, while the second (oxidation) step moved the interface beyond them. Conventional and high resolution cross-sectional electron microscopy showed pyramidal Si protrusions at the Si/SiO₂ interface under the grains. The size of the grains, as well as their distance from the Si/SiO₂ interface (peak of pyramids) can be controlled by the annealing process parameters. The process can be repeated and SiC nanocrystals (oriented in the same way) can be produced in a multilevel structure.     

Nanoparticle-Coated Silicon Nanowires

The synthesis of silicon nanowires that are densely coated with silicon nanoparticles is reported. These structures were produced in a two-step process, using a method known as hypersonic plasma particle deposition. In the first step, a Ti–Si nanoparticle film was deposited. In the second step the Ti-source was switched off, and nanoparticle-coated nanowires grew under the simultaneous action of Si vapor deposition and bombardment by Si nanoparticles. Total process time, including both steps, equaled 5 min, and resulted in formation of a dense network of randomly oriented nanowires covering1.5 cm² of substrate area. The nanowires are composed of single-crystal Si. The diameters of the nanowires vary over the range 100–800 nm. Each nanowire has a crystalline TiSi₂ catalyst particle, believed to have been solid during nanowire growth, at its tip.     

Growth of amorphous SiO<Subscript>2</Subscript> nanowires on Si using a Pd/Au thin film as a catalyst

Nanowires of amorphous SiO₂ were synthesized by thermal processing of a Si(100) substrate at 1100 °C in the presence of a nitrogen flow, and using a 15 nm thick high silicon-solubility Pd/Au film as a catalyst. The substrate itself was the only source of silicon for the nanowire growth. The nanostructures produced were characterized by high resolution transmission and scanning electron microscopy and by X-ray diffraction. The nanowire growth is consistent with the vapor-liquid-solid (VLS) mechanism, with particles of Pd₂Si and Au(Pd) being observed to form from the reaction between silicon and the catalytic film, and to remain at the tip of the wires. The synthesized nanowires showed a well defined morphology which could be very interesting for lasing applications. PACS 81.05.Ys; 81.10.Bk; 85.40.Ux     

Synthesis and microstructure of metal oxide thin films containing metal nanoparticles by liquid phase deposition (LPD) method

Au nanoparticles dispersed SiO₂-TiO₂ composite films have been prepared by a novel wet process, Liquid Phase Deposition (LPD) method. The composite films were characterized by XRD, XPS, TEM, ICP, SEM and UV-VIS absorption spectroscopy. The results showed that the SiO₂-TiO₂ composite films containing Au^III and Au^I ionic species were co-deposited from a mixed solution of ammonium silicofluoride, ammonium hexafluorotitanate, boric acid and tetrachloroauric acid. The heat treatment induced the reduction of Au ions and formation of Au nanoparticles in the film. TEM observation revealed that the Au nanoparticles with 5-10 nm in diameter were found to be dispersed uniformly in the SiO₂-TiO₂ matrix. The optical absorption band due to the surface plasmon resonance of dispersed Au particles were observed at the wavelength of 550 nm and shifted toward longer wavelength with increasing heat treatment temperature. Received 28 November 2000     

Synchrotron-Radiation Photoemission Study of Growth and Stability of Au Clusters on Rutile TiO2(110)-1×1

利用同步辐射高分辨光电子能谱研究了金团簇在部分还原TiO₂-(1×1)表面的生长和稳定性．价带谱实验结果观察到非常少量金团簇的沉积导致了Ti3+的3d峰完全消失,表明金团簇成核在TiO₂-(1×1)表面的氧缺陷位．Au4f芯电子光电子能谱实验结果证明了TiO₂-(1×1)表面氧缺陷位向金团簇转移电荷．还对比研究了化学剂量比和部分还原的TiO₂-(1×1)表面上金团簇的热稳定性．当金团簇尺寸相近时部分还原的TiO₂-(1×1)表面上金团簇要比化学剂量比的TiO₂-(1×1)面上金团簇稳定;在相同的表面上尺寸大的金团簇要比尺寸小的金团簇稳定．     

Formation of nanoparticles containing zinc in Si(001) by ion-beam implantation and subsequent annealing

The formation of nanoparticles containing zinc in Si(001) substrates by the implantation of ⁶⁴Zn⁺ ions and subsequent annealing in dry oxygen at 800 and 1000°C for 1 h is studied. The structure of the samples is studied by high-resolution transmission electron microscopy, X-ray diffraction, and photoluminescence spectroscopy. 20-nm zinc nanoparticles located at a depth of about 50 nm are revealed in the as-implanted sample. 10–20-nm pores are observed in the surface layer. Annealing leads to oxidation of the Zn nanoparticles to the Zn₂SiO₄ state. It is shown that the oxidation of Zn nanoparticles begins on their surface and at an annealing temperature of 800°C results in the formation of nanoparticles with the “соre–shell” structure. The X-ray diffraction technique shows simultaneously two Zn and Zn₂SiO₄ phases. ZnO nanoparticles are not formed under the given implantation and annealing conditions.     

Size-selected Au clusters deposited on SiO2/Si: Stability of clusters under ambient pressure and elevated temperatures

This study examined the oxidation and reduction behavior of mass-selected Au clusters consisting of 2-13 atoms deposited on silica. An atomic oxygen environment was used for the oxidation of Au. X-ray photoelectron spectroscopy (XPS) was used to identify Au(III) and Au(O). Au₅, Au₇ and Au₁₃ clusters deposited on the as-prepared SiO₂/Si substrates were highly inert towards oxidation, whereas the other clusters could be oxidized, i.e. the chemical property drastically changed with the number of atoms in a cluster. The size-selectivity in chemical reactivity remained unchanged upon air-exposure. The chemical properties of the deposited Au clusters were unchanged after annealing at 250 °C. Annealing at higher temperatures caused structural changes to the surface, as determined by the oxidation behavior. XPS of the deposited Au clusters upon annealing indicated charge transfer from Au to silica.