单晶ZnO纳米线的合成和生长机理研究

用化学气相输运(CVT)方法合成了直径在20～120nm呈单晶结构的ZnO纳米线.利用场发射扫描电 镜(FESEM)、高分辨透射电镜(HRTEM)以及选区电子衍射(SAED)等技术对ZnO纳米线的生长机理和结构进行 了系统研究,结果表明,纳米线的成核与Au Zn合金催化颗粒的饱和度有直接的关系,先饱和的颗粒上纳米线首 先成核.纳米线顶端合金颗粒组成的变化是导致纳米线生长终止的重要原因,大量纳米线的生长不是同时进行 的.本工作提供了支持纳米线气液固(V L S)生长机理的新实验证据,提出了氧化物纳米线的生长机理.     

Electrochemically grown rough-textured nanowires

Nanowires with a rough surface texture show unusual electronic, optical, and chemical properties; however, there are only a few existing methods for producing these nanowires. Here, we describe two methods for growing both free standing and lithographically patterned gold (Au) nanowires with a rough surface texture. The first strategy is based on the deposition of nanowires from a silver (Ag)–Au plating solution mixture that precipitates an Ag–Au cyanide complex during electrodeposition at low current densities. This complex disperses in the plating solution, thereby altering the nanowire growth to yield a rough surface texture. These nanowires are mass produced in alumina membranes. The second strategy produces long and rough Au nanowires on lithographically patternable nickel edge templates with corrugations formed by partial etching. These rough nanowires can be easily arrayed and integrated with microscale devices.     

Synthesis and thermal stability of gold nanowires within monolithic mesoporous silica

This paper describes the preparation, by a novel and simple method, and the thermal stability of gold nanowires within monolithic mesoporous silica, involving soaking monolithic mesoporous silica in HAuCl₄ aqueous solution, followed by drying and subsequent step-annealing. It has been shown that reduction of Au³⁺ within silica pores can occur during the drying process at 80 °C without any special reduction treatment. After initial annealing at 300 °C, Au nanowires are formed within the pores and are stable at temperatures up to 500 °C. Increasing the annealing temperature leads to a wire-to-rod-to-sphere morphological transformation of the Au nanowires. The surface-mediated reducing groups (-OH) on the silica pore are responsible for the low-temperature reduction of Au³⁺ ions, and the formation of Au nanowires is attributed to the uni-directional diffusion of Au atoms and the confinement of the pore channels. Spheroidization and breaking at some defects in the Au nanowires during annealing at elevated temperature result in the wire-to-rod-to-sphere transformation, accompanied by a blue-shift of the surface plasmon resonance over a very wide region in the optical spectrum. PACS 81.07.-b; 81.40.-z; 81.05.Rm     

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO₄ tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO₄ tetrahedra are readily formed even at room temperature. Nevertheless, the SiO₂ thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ～400°C), silicon diffusion towards the surface is promoted and a thicker SiO₂ layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.     

Room temperature facile synthesis of CuO nanostructures by resistive heating

CuO porous layer and nanowires were in situ grown on Cu wires by a very simple catalyst-free thermal oxidation process based on resistive heating of pure metal wires at ambient conditions. The morphological, compositional and structural characterization of the obtained samples revealed that the nanowires are monoclinic single and bi-crystalline structures with mean diameters of 90–300 nm and typical length in the range 1–5 μm. The effects of the applied voltage values and treatment time on the morphology of the metal substrate and on the size of the nanowires were investigated. Different from the vapor–solid (V–S) mechanism, the growth of nanowires is found to be based on the Cu ion diffusion.     

Investigation of the surface properties of gold nanowire arrays

Gold nanowire arrays with diameters ranging from 45 to 200 nm were obtained via electrochemical deposition within the ion-track templates. The morphology of gold nanowires was imaged by scanning electron microscopy (SEM). The surface properties were investigated by surface plasmon resonance (SPR) and X-ray photoelectron spectroscopy (XPS). The SPR peaks were observed as the gold nanowire arrays embedded in the templates and their intensity decreased after the sample exposed to the air for a certain time due to the formation of chemisorbed oxygen on nanowire surface. The positive binding energy shifts in Au core level was found when the gold nanowire arrays embodied in template and the initial- and finial-state effects were introduced to explain this phenomenon.     

Morphology control of the fabricated hydrophobic gold nanostructures in W/O microemulsion under microwave irradiation

The hydrophobic gold nanostructures with different shapes were prepared by using n-butanol situ reduction in cetyltrimethylammonium bromide (CTAB)/n-butanol/n-heptane/HAuCl₄(aq) W/O microemulsion through microwave dielectric heating. The CTAB-stabilized gold nanoparticles and nanowires with a networked structure obtained through this method were characterized and analyzed by ultraviolet visible spectroscopy (UV-vis), transmission electron microscopy (TEM) and X-ray diffraction (XRD), respectively. The morphologies of the hydrophobic gold nanocrystals were successfully controlled through the adjustment of molar ratios of CTAB/HAuCl₄ in the reverse microemulsion. The formation mechanism of anisotropic gold nanostructures was discussed, which demonstrated that CTAB played an important role in forming and stabilizing the shape of gold nanowires.     

Carbon films grown on Pt(1 1 1) as supports for model gold catalysts

Carbon films were grown on a Pt(1 1 1) single crystal by ethylene decomposition at elevated temperatures (1000-1300 K). Depending on the preparation conditions, different carbon structures formed on the metal surface such as flat and curved graphitic layers, carbon particles and carbon nanowires. Although these carbon films exhibited a high density of surface defects, gold interacted only weakly with the carbon surface. CO adsorption on the Au/carbon systems was very similar to that observed for various Au/oxide systems previously studied. This finding strongly indicates that CO adsorption on gold is essentially independent of the nature of support.     

Photochemical Synthesis and Properties of Colloidal Copper,Silver and Gold Adsorbed on Quartz

Original methods for the photochemical production of stable copper, silver and gold colloids in the form of films on quartz, and dispersion in liquids were devised. It is shown that photochemical synthesis of colloidal metals is a difficult multiphase process, and includes the formation of low-valence forms of Cu(I), Au(I) and nonmetal clusters, colloidal particles and their agglomerates. Cluster stabilization and further growth to colloidal particles are achieved by adsorption onto the solid surface (quartz) or by increasing the viscosity of photolyte. In the absence of these methods of stabilization, the processes of intermediate reoxidation to Cu(II) and Au(III) and agglomeration of Ag and Au colloids proceed in a photolyte. Adsorption and the rate of cluster growth on a quartz surface are speeded up by the action of monochromatic UV light. Experimental models of the mechanism of colloidal formation are suggested. The dependence of the growth rate and the properties of the colloids on conditions of the photochemical procedure (energy and light intensity, concentration of initial complex) has been established.     

AFM study of gold nanowire array electrodeposited within anodic aluminum oxide template

Uniform gold nanowires were synthesized by electrodepositing the gold under a very low ac frequency in the pores of an anodic aluminum oxide (AAO) template. The surface of the Au/AAO composite is very even and appeared purplish red. Atomic force microscopy observation indicates that the template membranes we obtained have hexagonally close-packed nanochannels. The gold nanowire array is very orderly arranged after partially dissolving the aluminum oxide membrane. Gold nanowires were also characterized by transmission electron microscopy and the phase structure of the Au/AAO composite was proved by X-ray diffraction. Received: 19 April 2001 / Accepted: 28 April 2001 / Published online: 27 June 2001     

Vernier effect of cascaded dual microring sensor

Recently, gold nanorods (Au NRs) have attracted much attention because at a particular photoelectricity the gold nanorods present a characteristic which is different from other types of Au nanomaterials with various shapes. Accurate measurement of aspect ratios does provide very high value of optical property for Au NRs. Monte Carlo (MC) simulation is thought of as the most accurate tool to perform size measurement through extracting structure parameters from the simulated scanning electron microscopy (SEM) image which best matches the experimental one. In this article, a series of MC-simulated secondary electron (SE) images have been taken for Au NRs on a silicon substrate. However, it has already been observed that the two ends of Au NRs in the experimental SEM image is brighter than that of the middle part. It seriously affects the accuracy of size measurement for Au NRs. The purpose of this work is to understand the mechanism underlying this phenomenon through a series of systematical analysis. It was found that the cetyltrimethylammonium bromide (CTAB) which covers the Au NRs indeed can alter the contrast of Au NRs compared to that without CTAB covering. However, SEs emitting from CTAB are not the reason for the abnormal brightness at the two ends of NRs. This work reveals that the charging effect might be the leading cause for this phenomenon.     

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.     

Stability of gold nanostructures on rutile TiO2(110) surface

The adsorption of gold atoms and formation of nanostructures on the rutile TiO₂(110) surface with different degree of oxygen reduction was studied from first principles. The Au atoms adsorb strongest at oxygen vacancy sites. Starting from a very low coverage limit the potential energy profiles or diffusion paths of the adsorbed Au monomers and dimers were calculated. Stable structures of two to nine Au atoms arranged in finite and infinite rows and in the shape of finite-size clusters were determined. All these structures are found to bind to the reduced surface stronger than 2 eV/atom. The elongated Au row-like structures bind by about 0.1 eV stronger than 3D clusters, suggesting a preference for the 1D-like Au growth mode on the missing-row reconstructed TiO₂(110).     

Photoluminescence of gold, copper and niobium as a function of temperature

A specially constructed instrument for measuring the low intensity photoluminescence emission spectra of metals is described. It uses low luminescence optical components and dedicated sample mounting techniques. Room temperature measurements agree closely with literature spectra for high-purity gold and are found to be sensitive to 100 ppm impurities. Detailed spectra are presented, which are weakly temperature dependent, for gold, copper and unpolished niobium between room temperature and 100 K. We conclude that this work provides accurate luminescence data for Au from 300 K down to 100 K. Although the (variable temperature) luminescence data for Cu are consistent both with the room temperature experimental data in the literature and theory, we conclude the role of surface adsorbates and/or oxides cannot be ruled out. Theory suggests that Nb has a factor ∼50 lower luminescence intensity than Au and Cu because the real part of the refractive index is a factor ∼5 higher and the density of states ∼2 eV below the Fermi energy is a factor of ∼4 lower than Au and Cu. Measurements are presented for unpolished Nb, but given the lack of signal detection for polished Nb and that theory predicts very weak signals, we conclude that the luminescence signals from pure Nb still remain below the sensitivity of our instrument.     

X射线光电子能谱法研究硅纳米线的能带结构

制备了氧辅助热分解法,以一氧化硅为原料,以氩气为载气,维持管内压强为1000Pa,在高温炉中于1250℃下反应5 h后得到硅纳米线。硅纳米线经5%氢氟酸水溶液处理5 min后,与1×10-3 mol·L-1的氯化金溶液中反应5 min,在硅纳米线的表面上修饰了金纳米粒子,用X射线粉末衍射表征了产物的结构,同时观察到单质硅和金的XRD图谱;用电子扫描和透射显微镜观察了产物的形貌,表明氧辅助方法可制得大量均匀的硅纳米线,修饰在硅纳米线上的金纳米点形状整齐,尺寸均匀,平均直径约8 nm;并用X射线光电子能谱分析了修饰过程中能带结构的变化。结果表明,金纳米粒子表面带负电,它在施主能级和受主能级上都有电子存在;由于氧杂质的存在,硅纳米线的费米能级移向价带顶。     

Catalytic and non-catalytic growth of amorphous silica nanowires and their photoluminescence properties

A simple method based on the thermal oxidation of Si wafers in presence of a mixture of MgO and graphite powder was developed for large-scale synthesis of very long amorphous silica nanowires. The synthesis was done with and without gold as the catalyst. Almost aligned uniform nanowires with diameters within 60-90 nm and length up to few hundred micrometers were obtained using gold as the catalyst while bicycle chain like nanowires were obtained in absence of the catalyst. The growth sequence of the nanowires was observed through scanning electron microscope. Both forms of the nanowires emitted blue lights at 414 nm (3 eV) under excitation at 250 nm.     

Low temperature growth and dimension- dependent photoluminescence efficiency of semiconductor nanowires

Low temperature growth and dimension dependent photoluminescence (PL) efficiency of semiconductor nanowires were investigated with CdS as a model system. The CdS nanowires were prepared with a simple, low temperature metal-organic chemical vapor deposition (MOCVD) process via the vapor–liquid–solid (VLS) mechanism. The low growth temperature of 360 °C was made possible with a newly developed single-source precursor of CdS and by using sputtered Au as the catalyst for the VLS growth. The length and diameter of the nanowires were adjusted by reaction time and sputtering conditions of Au, respectively. Nanowires of up to several μm in length and 20 to 200 nm in diameter were obtained. The PL quantum yield of the nanowires was found to decrease with increasing wire length, but to increase with decreasing wire diameter. This dimension-dependent PL efficiency of one-dimensional nanostructure, unlikely resulting from the quantum size confinement effect, appears to be a new observation that carries application significance. PACS 74.25.Gz; 78.55.Et; 78.67.Lt     

Synthesis and characterization of electrodeposited permalloy (Ni80Fe20)/Cu multilayered nanowires

Permalloy (Ni₈₀Fe₂₀)/Cu multilayered nanowires (NWs) were electrodeposited using a template directed method from sulfate baths via pulse potential technique. Microstructures and compositions of the nanowires were characterized using various microscopy and spectroscopy techniques. To synthesize compositionally uniform nanowires with high efficiency, new sulfate baths with a high content of Ni²⁺ were developed. The effects of deposition potential and concentration of metal ions were optimized to reduce composition inhomogeneity and incorporation of copper in the permalloy layers. Composition of the NiFe layers was found to be close to 20 at% Fe with a maximum of 5 at% Cu. TEM analysis indicated that individual nanowires exhibit distinct and coherent layering structure with rough and wavy interfaces. A synthesized single nanowire was also AC dielectrophoretically assembled across the microfabricated gold electrodes for subsequent magnetoresistance measurements.     

Interaction of carbon monoxide and oxygen at the surface of inverse titania/Au model catalyst

Interaction of carbon monoxide and oxygen on the surface of titania/Au(1 1 1) inverse model catalyst held at 200 K has been studied by reflection absorption infrared spectroscopy. It was found that CO adsorbs on the oxide/Au perimeter interface, whereas no or very weak adsorption was observed on Au(1 1 1) or titania surface, respectively. Exposing of such species to oxygen results in their decay possibly due to carbon dioxide formation. Efficiency of this effect is higher at lower CO initial concentration which points at the importance of free surface sites for the reaction process.     

Facile synthesis and electrochemical properties of octahedral gold nanocrystals

High-yield octahedral gold nanocrystals of ~45 nm in size have been facilely synthesized by one-pot reduction of HAuCl₄ using formic acid in cetyltrimethylammonium bromide (CTAB) aqueous solution. The results showed that CTAB can promote the formation of single-crystalline nucleation and preferentially adsorb on the (111) planes of gold nanocrystals, resulting in the formation of octahedral gold nanocrystals. Formic acid acted as not only a mild reducing agent, but also could promote the formation of (111) facet. The octahedral gold nanocrystals exhibited similar cyclic voltammetry (CV) curves to single-crystal Au (111) electrode and excellent electrocatalytic activity for methanol oxidation. This synthetic strategy may open new route for facile synthesis of shape-controlled metal nanoparticles.