Modelling self-ordering behaviour on Ag covered Pt vicinal surfaces

We use computer simulation to explore the formation process of a monolayer of Ag on a stepped Pt(111) substrate and the formation of 3D Pt nanostructures on an Ag covered (111) and (100) Pt substrate. We show that broken lines of Pt nanostructures are preferred at the step edges on the (111) substrate while continuous lines of Pt nanowires are preferred at the step edge on the (100) substrate. This different behaviour is due to the exposed front facet of the nanostructures running along the step, specifically for the (100) stepped substrate a nanowire grown on the step edge has a stable (111) exposed front facet, whereas a nanowire grown on the (111) substrate would have an unstable (100) front facet (depending on the direction of the step). For the Pt nanowires grown on the (100) substrate we show how arriving Pt dimers (and monomers) preferentially move up off the Ag substrate onto the nanowire's (111) facet where they undergo fast diffusion. We also show that these Pt dimers (and monomers) move up and down the nanowire's facet until a vacancy or defect is encountered.     

3D-assembled Ag nanowires for use in plasmon-enhanced spectroscopic sensors

Surface plasmon-enhanced spectroscopic sensors for fluorescence and Raman spectroscopy use high-density metallic nanostructures to strongly enhance the light–matter interaction. In this contribution, we will review the processes by which three-dimensional (3D) multilayered Ag nanowires assemble from one-dimensional Ag nanowires and their plasmon-enhanced sensing applications, giving emphasis to the physical mechanism underlying the plasmon-enhanced spectroscopy. In particular, we discuss the practical aspects of 3D porous and flexible plasmonic platforms used for spectroscopic sensing applications. Combining a portable spectrometer with a low-cost but highly sensitive and flexible plasmonic substrate is potentially useful for on-site chemical analysis in the contexts of environmental monitoring, food safety, forensic science, and point-of-care healthcare medical diagnostics.     

Nanoporous Ag–GaN thin films prepared by metal‐assisted electroless etching and deposition as three‐dimensional substrates for surface‐enhanced Raman scattering

Three‐dimensional (3D) nanoporous gallium nitride (PGaN) scaffolds are fabricated by Pt‐assisted electroless hydrofluoric acid (HF) etching of crystalline GaN followed by in situ electroless deposition of Ag nanostructures onto the interior surfaces of the nanopores, yielding a large surface area substrate for surface‐enhanced Raman scattering (SERS). The resulting 3D SERS‐active substrates have been optimized by varying reaction parameters and starting material concentration, exhibiting enhanced Raman signals 10–100× more intense than either (1) sputtered Ag‐coated porous GaN or (2) Ag‐coated planar GaN. The increase in SERS signal is attributed to a combination of the large surface area and the inherent transparency of PGaN in the visible spectral region. Overall, Ag‐decorated PGaN is a promising platform for high sensitivity SERS detection and chemical analysis, particularly for reaction and metabolic products that can be trapped inside the highly anisotropic nanoscale pores of PGaN. The potential of this sampling mode is illustrated by the ability to acquire Raman spectra of adenine down to 5 fmol. Additionally, correlated SERS and laser desorption/ionization mass spectrometry spectra can be acquired from same sample spot without further preparation, opening new possibilities for the investigation of surface‐bound molecules with substantially enhanced information content. Copyright © 2012 John Wiley & Sons, Ltd.     

Two‐Dimensionally Ordered Plasmonic and Magnetic Nanostructures on Transferable Electron‐Transparent Substrates

Discovery of new plasmonic behaviors from nanostructured materials can be greatly accelerated by the ability to prepare and characterize their near‐field behaviors with high resolution in a rapid manner. Here, an efficient and cost‐effective way is reported to make 2D periodic nanostructures on electron‐transparent substrates for rapid characterization by transmission electron microscopy. By combining nanosphere lithography with a substrate float‐off technique, large areas of electron‐transparent periodic nanostructures can be achieved. For this study, the synthesis of plasmonic nanostructures of Ag, magnetic nanostructures of Co, and bimetallic nanostructures of Ag–Co are investigated. Characterization of the materials by a combination of transmission electron microscopy, far‐field optical spectroscopy, and magnetization measurements reveals that this new approach can yield useful nanostructures on transparent, flexible, and transferable substrates with desirable plasmonic and/or magnetic properties.     

Atom lithography with a holographic light mask

In atom lithography with optical masks, deposition of an atomic beam on a given substrate is controlled by a standing light-wave field. The lateral intensity distribution of the light field is transferred to the substrate with nanometer scale. We have tailored a complex pattern of this intensity distribution through diffraction of a laser beam from a hologram that is stored in a photorefractive crystal. This method can be extended to superpose 1000 or more laser beams. The method is furthermore applicable during growth processes and thus allows full 3D structuring of suitable materials with periodic and non-periodic patterns at nanometer scales.     

Self-assembled triangular and labyrinth buckling patterns of thin films on spherical substrates

We investigated the possibility of controlling thin film buckling patterns by varying the substrate curvature and the stress induced therein upon cooling. The numerical and experimental studies are based on a spherical Ag core/SiO(2) shell system. For Ag substrates with a relatively larger curvature, the dentlike triangular buckling pattern comes out when the film nominal stress exceeds a critical value. With increasing film stress and/or substrate radius, the labyrinthlike buckling pattern takes over. Both the buckling wavelength and the critical stress increase with the substrate radius.     

Ag nanostructures on Au(7 8 8): A self-assembled superlattice of metallic quantum resonators

We have studied by scanning tunneling microscopy (STM) the effect of the reconstruction of a stepped Au(1 1 1) surface on the growth of silver sub-monolayer deposition. For narrow terraces, the reconstruction is disturbed and its pattern changes, Ag growth is therefore influenced. Thus growth of Ag on Au(7 8 8) vicinal surface can be controlled and leads to the formation of a highly ordered superlattice of nanostructures. Moreover, we show by tunneling conductance images that Ag islands exhibit electronic confinement effects of the Shockley surface state. Due to the homogeneity of their shapes and sizes, all the nanostructures of the self-assembled superlattice should exhibit similar electronic properties.     

Hydrogen on hybrid MoS2/graphene nanostructures

Transition metal dichalcogenides are rising candidates for the replacement of Pt catalysts in water splitting. In this theoretical study we focus on the hydrogen evolution reaction part of this process and on how hydrogen (H) interacts with MoS₂ nanostructures, free‐standing or positioned on a graphene substrate. Density functional theory calculations confirm the stability of such nanostructures and our results for H on several configurations, from 2D infinite monolayers to quasi‐1D MoS₂ ribbons and quasi‐0D MoS₂ flakes, are presented. We calculate the adsorption energy of H atoms on various sites of the MoS₂ nanostructures, notably at Mo and S active edges. Comparing free‐standing and MoS₂/graphene hybrid systems we find that the effect of the support on the adsorption of H on MoS₂ nanostructures is quite significant when the substrate induces strain. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Profile calculation of gold nanostructures by dispersed-nanosphere lithography through oblique etching for LSPR applications

Nanosphere lithography is an inexpensive method used to fabricate gold nanostructures on a substrate. Using dispersed-nanosphere lithography, in which the nanospheres are dispersed on a substrate, 2D or 3D nanostructures can be fabricated by obliquely depositing a gold film on the nanospheres and etching the gold film afterward. These nanostructures are tunable and acute, and are thus good emitting elements for the localized surface plasmon resonance applications. So far, for the fabrication of nanostructures on a substrate with dispersed nanospheres, only 2D nanostructures have been reported through perpendicular etching. We report in this paper that the 3D nanostructures fabricated by dispersed-nanosphere lithography are rigid non-conformal structures, and perpendicular gold etching can be expanded to oblique etching, which provides more possibilities for fabricating the gold nanostructures in various shapes. The profiles of gold nanostructures after several varying angle depositions, and their final profiles after perpendicular or oblique etching, are calculated in this paper. Our profile simulations are applicable for nanospheres (or microspheres) within the range of tens of nanometers to tens of micrometers, and are consistent with our fabricated nanostructures observed using scanning electron and atomic force microscopy. Electronic Supplementary Material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Microwave-assisted synthesis of silver nanocrystals in benzyl alcohol and their subsequent in situ chemical transformation into Ag–AgCl nanohybrids for plasmonic photocatalysis

In heterostructured metal–semiconductor system, plasmonic metal nanostructures can cooperate with semiconductors to enhance the solar light harness and energy conversion. In this study, we report on the plasmonically photocatalytic system constructed by in situ growth of semiconductor on metal nanostructures with high performance and stability. A facile and rapid microwave-assisted route was first explored to synthesize Ag nanocrystals, and subsequently converting them to Ag–AgCl nanohybrids was realized by in situ chemical transformation strategy at room temperature. These Ag–AgCl nanohybrids were characterized by X-ray diffraction, scanning electronic microscopy, and UV–vis absorption spectroscopy. The resulting Ag–AgCl nanohybrids showed remarkable photocatalytic activity and excellent durability for the degradation of organic pollutants under visible light irradiation. This finding provides a new way to improve photocatalytic efficiency through controllable chemical transformation.     

ZnO three-dimensional hedgehog-like nanostructure: synthesis,growth mechanism and optical enhancement

The 3D hedgehog-like ZnO nanostructures were synthesized on Si substrate through chemical vapor deposition process. The morphology and structure of the products were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, as well as transmission electron microscopy. The ZnO 3D hedgehog-like architectures were found to consist of a central nucleus and multiple side-growing nanowires with diameter of 100–250 nm and length up to 10 µm. The growth mechanism of the hedgehog-like ZnO nanostructures was studied. It revealed a three-step process during the entire growth. Finally, room temperature photoluminescence spectra of ZnO 3D nanostructures showed that the center excitation would render much stronger PL emission intensity. Furthermore, simulation results indicated that the enhanced emission came from light-trapping-induced excitation light field enhancement.     

Study of nanoclusters growth at initial stages of ultrathin film deposition by kinetic modeling

Comparative analysis of Au, Cu, Pt, Ni and Fe nanoclusters growth on amorphous carbon substrate by proposed kinetic model based on rate equations is present. Partial sticking coefficients introduced into the model let to discriminate elementary processes such as adatom adsorption and diffusion on bare substrate and on top of islands, nucleation and mobility of islands and its coalescence, 2-d and 3-d island growth modes. The quantitative fittings of experimental time dependencies of surface coverage, clusters density, cluster size are performed by solving model equations. From the best fittings the values of phenomenological coefficients defining elementary processes are found for different materials. Comparative analysis of those coefficients let to discover mechanisms of nanoclusters formation and growth of different materials. It is shown that clusterization for Cu and Au is more favorable than for Pt and Ni. Diffusivity for Pt and Ni on amorphous carbon (a-C) substrate is significantly less than for Au and Cu. In opposite, diffusivity on the top of islands for Ni and Pt is significantly higher than for Au and Cu. The mobility of islands for Au and Cu is much higher than for Ni and Pt. The fitting of experimental curves of Fe deposition on a-C at different temperatures showed that temperature mainly influences sticking process but not diffusion.     

Initial growth process and surface structure of Ag on Si(111) studied by low-energy Ion-Scattering Spectroscopy (ISS) and LEED-AES

The growth process of silver on a Si(111) substrate has been studied in detail by low-energy ion-scattering spectroscopy (ISS) combined with LEED-AES. Neon ions of 500 eV were used as probe ions of ISS. The ISS experiments have revealed that the growth at room temperature and at high temperature are quite different from each other even in the submonolayer coverage range. The following growth models have been proposed for the respective temperatures. At room temperature, the deposited Ag forms a two-dimensional (2D) island at around 2/3 monolayer (ML) coverage, where the Ag atoms are packed commensurately with the Si(111)1 substrate. One third of the substrate Si surface remains uncovered there. Then it starts to develop into Ag crystal, and at a few ML coverage a 3D island of bulk Ag crystal grows directly on the substrate. An intermediate layer, which covers uniformly the whole surface before the growth of Ag crystal, does not exist. At high temperatures (>~200°C), the well-known Si(111)√3-Ag layer is formed as an intermediate layer, which consists of 2/3 ML of Ag atoms and covers the whole surface uniformly. These Ag atoms are embedded in the first double layer of the Si substrate. It is concluded that the formation of the √3 structure needs relatively high activation energy which may originate from the large displacement of Si atoms owing to the embedment of the Ag atoms, and does not proceed below about 200°C. The most stable state of the Ag atoms on the outermost Si layer is in the shape of an island, both for the Si(111) surface and for the Si(111)√3-Ag surface.     

Catalyst patterning methods for surface-bound chemical vapor deposition of carbon nanotubes

We present three different catalyst preparation and patterning techniques for plasma-enhanced chemical vapor deposition of carbon nanostructures from acetylene and ammonia mixtures. The different merits and potential areas of application are highlighted for each technique as compared to the benchmark of e-beam-lithography patterning. Maskless, focused ion beam written Pt can nucleate aligned carbon nanofibers, thereby allowing a sub-100 nm lateral resolution on non-planar substrate geometries combined with an in-situ monitoring. Ion beam milling additionally enables the pre-shaping and marking of the substrate, which is shown for the growth of individual nanofibers on the apex of commercial scanning probe tips. Pulsed electrochemical deposition was used to form Ni and Fe catalyst islands of controlled size and density. This is also demonstrated on complex substrate geometries such as carbon cloth. Nanocontact printing was employed to deposit a highly purified Co colloid in regular patterns with feature sizes down to 100 nm onto silicon wafers for low cost patterning over large areas. We analyze the catalyst restructuring upon exposure to elevated temperatures for each technique and relate this to the nucleated nanofiber dimensions and array densities. The flexibility in catalyst and substrate material allows a transfer of our achievements to catalyst-assisted growth of nanostructures in general facilitating their hierarchical device integration and future application. PACS 81.16.Rf; 81.16.Hc; 61.46.+w     

Three-dimensional noble-metal nanostructure: A new kind of substrate for sensitive,uniform, and reproducible surface-enhanced Raman scattering

Surface-enhanced Raman spectroscopy(SERS) is a powerful vibrational spectroscopy technique for highly sensitive structural detection of low concentration analyte. The SERS activities largely depend on the topography of the substrate.In this review, we summarize the recent progress in SERS substrate, especially focusing on the three-dimensional(3D)noble-metal substrate with hierarchical nanostructure. Firstly, we introduce the background and general mechanism of3 D hierarchical SERS nanostructures. Then, a systematic overview on the fabrication, growth mechanism, and SERS property of various noble-metal substrates with 3D hierarchical nanostructures is presented. Finally, the applications of 3D hierarchical nanostructures as SERS substrates in many fields are discussed.     

Flame synthesis of tungsten oxide nanostructures on diverse substrates

One-dimensional (1D) tungsten oxide nanostructures show great potential for applications in the areas of batteries, photoelectrochemical water-splitting, electrochromic devices, catalysts and gas sensors. 1D tungsten oxide nanostructures are currently synthesized by physical or chemical vapor deposition, which are limited by low temperatures, the need for vacuum conditions, frequently expensive catalysts, and difficulty in scaling up for mass-production. These limitations, however, can be overcome by flame synthesis. Here, using a co-flow multi-element diffusion burner, we demonstrate the atmospheric, catalyst-free, rapid, mild and scalable flame synthesis of diverse, quasi-aligned, large density, and crystalline tungsten oxide nanostructures on a variety of substrates. Specifically, under fuel-rich conditions, monoclinic 1D W₁₈O₄₉ nanowires and nanotubes were grown on tungsten, iron, steel and fluorinated tin oxide (FTO) substrates, with controlled diameters ranging from 10 to 400 nm and axial growth rates ranging from 2 to 60 μm/h. Monoclinic 1D WO₃ nanowires and nanotubes were grown, instead, on silicon and silicon dioxide substrates. Under fuel-lean conditions, diverse WO₃ nanostructures, including monoclinic 1D nanowires, cubic 2D nanobelts and monoclinic 3D nanocones were grown on tungsten and FTO substrates. The success of this versatile flame synthesis method is attributed to the large tunability of several synthesis parameters, including the flame stoichiometry, the tungsten source and growth substrate temperatures, the tungsten oxide vapor concentration, and the material of the growth substrate. This flame synthesis method can be extended to synthesize other 1D transition metal oxides as well, enabling many large-scale electronic and energy conversion applications.     

Ag在Si(111)表面二维生长有序化动力学研究

利用自制的高分辨率衍射斑角分布测量仪对低能电子衍射斑进行测量,研究Ag在Si(111)表面的有序化规律.结果表明,Ag3^1/2畴在生长过程中,满足标度不变性,是一种自相似的生长过程,在生长初期,它的生长指数近似为1/2,并提出了Ag从Ag(111)岛向外扩散形成Ag3^1/2畴的生长机制.还提出利用光助退火以在较低温度下获得完善的表面结构.     

A multitechnique surface science examination of Sn deposition on Pt(100)

Interfaces prepared by vapor deposition of Sn onto Pt(100) surfaces have been examined using the following techniques: Auger electron and X-ray photoelectron spectroscopy (AES and XPS), low-energy electron diffraction (LEED), and low-energy ion surface scattering (LEISS) with Ne⁺ ions. Tin deposition was conducted at 320 and 600 K, and the surface composition and order was examined as a function of further annealing to 1200 K. The AES uptake plots (signal versus deposition time) indicate that the Sn growth mode can be described by a layer-by-layer process only up to one adayer at 320 K. Some evidence of 3D growth is inferred from LEED and LEISS data for higher Sn coverages. For deposition at 600 K, AES data indicate significant interdiffusion and surface alloy formation. LEED observations (recorded at a substrate temperature of 320 K) show that the characteristic hexagonal Pt(100) reconstruction disappears with Sn exposures of 4.6 × 10¹⁴ atoms cm² (θ_Sn = 0.35 monolayer (ML)). Further Sn deposition results in a c(2 × 2) LEED pattern starting at a coverage of slightly above 0.5 ML. The c(2 × 2) LEED pattern becomes progressively more diffuse with increasing Sn exposure with eventual loss of all LEED features above 2.2 ML. Annealing experiments with various precoverages of Sn on Pt(100) are also described by AES, LEED, and LEISS results. For specific Sn precoverages and annealing conditions, c(2 × 2), p(3√2 × √2)R45°, and a combination of the two LEED patterns are observed. These ordered LEED patterns are suggested to arise from ordered PtSn surface alloys. In addition, the chemisorption of CO and O₂ at the ordered annealed Sn/Pt(100) surfaces was also examined using thermal desorption mass spectroscopy (TDMS), AES, and LEED.     

Improving light trapping and conversion efficiency of amorphous silicon solar cell by modified and randomly distributed ZnO nanorods

Three-dimensional （3D） nanostructures in thin film solar cells have attracted significant attention due to their appli- cations in enhancing light trapping. Enhanced light trapping can result in more effective absorption in solar cells, thus leading to higher short-circuit current density and conversion efficiency. We develop randomly distributed and modified ZnO nanorods, which are designed and fabricated by the following processes： the deposition of a ZnO seed layer on sub- strate with sputtering, the wet chemical etching of the seed layer to form isolated islands for nanorod growth, the chemical bath deposition of the ZnO nanorods, and the sputtering deposition of a thin Al-doped ZnO （ZnO：Al） layer to improve the ZnO/Si interface. Solar cells employing the modified ZnO nanorod substrate show a considerable increase in solar energy conversion efficiency.     

Effect of different catalysts and growth temperature on the photoluminescence properties of zinc silicate nanostructures grown via vapor-liquid-solid method

We explore the photoluminescence properties of zinc silicate (Zn₂SiO₄) nanostructures synthesized by vapor-liquid-solid (VLS) mode of growth using three different catalysts (Sn, Ag, and Mn). Different catalysts significantly influence the growth rate which in turn has an impact on the structure and hence the photoluminescence of the prepared zinc silicate nanostructures. Zn₂SiO₄ has a wide bandgap of about 5.5 eV and in its pure form, it does not emit in visible region due to its inner shell electronic transitions between the 3d⁵ energy levels. However, the incorporation of different catalysts (Sn, Ag and Mn) at different growth temperatures into the Zn₂SiO₄ crystal growth kinetics provides wide visible spectral range of photoluminescence (PL) emissions. PL analysis shows broad multi-band spectrum in the visible region and distinct colors (red, yellow, green, blue, cyan and violet) are obtained depending on the crystalline structure of the prepared nanostructures. The allowed transitions due to the effect of different catalysts on zinc silicate lattice offer a huge cross-section of absorption that generates strong photoluminescence. The correlation between the structural and optical properties of the synthesized nanostructures is discussed in detail. The synthesized photoluminescent nanostructures have potential applications in solid-state lighting and display devices.