Kinetically controlled synthesis of triangular and hexagonal nanoplates of palladium and their SPR/SERS properties

The rapid reduction of Na(2)PdCl(4) by ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP) has recently been demonstrated as a convenient method of generating Pd cubooctahedra and twinned nanoparticles. Here we describe a new procedure where Pd triangular or hexagonal nanoplates could be selectively synthesized by manipulating the reduction kinetics of the polyol process. More specifically, the reduction rate was substantially reduced through the introduction of Fe(III) species and the O(2)/Cl(-) pair, two wet etchants for Pd(0). The etching power of the O(2)/Cl(-) pair could be further enhanced by adding an acid to lower the pH of the reaction solution. Unlike the previously reported synthesis of Ag and Au nanoplates, light was found to have no indispensable role in the formation of Pd nanoplates. Both triangular and hexagonal nanoplates of Pd exhibited surface plasmon resonance (SPR) peaks in the visible region, and their positions matched with the results of discrete dipole approximation (DDA) calculation. Thanks to their sharp corners and edges, these Pd nanoplates could serve as active substrates for surface-enhanced Raman scattering (SERS).     

Starfruit-shaped gold nanorods and nanowires: synthesis and SERS characterization

Recently, branched and star-shaped gold nanoparticles have received significant attention for their unique optical and electronic properties, but most examples of such nanoparticles have a zero-dimensional shape with varying numbers of branches coming from a quasi-spherical core. This report details the first examples of higher-order penta-branched gold particles including rod-, wire-, and platelike particles which contain a uniquely periodic starfruitlike morphology. These nanoparticles are synthesized in the presence of silver ions by a seed-mediated approach based on utilizing highly purified pentahedrally twinned gold nanorods and nanowires as seed particles. The extent of the growth can be varied, leading to shifts in the plasmon resonances of the particles. In addition, the application of the starfruit rods for surface-enhanced Raman spectroscopy (SERS) is demonstrated.     

钯纳米颗粒形貌控制合成及其SPR／SERS性质

在水溶液中分别以十六烷基三甲基溴化铵（CTAB）和CTAB／柠檬酸钠混合剂为包覆剂合成钯纳米颗粒,并研究其形貌演变．钯纳米颗粒在成核阶段会形成具有不同孪晶结构的晶核,在生长阶段又会选择性的放大某一组晶面,这两个因素导致了钯纳米颗粒形貌的多样性．在合成中CTAB既会影响钯纳米颗粒的成核,也会影响颗粒晶面的选择性生长．通过改变CTAB和还原剂的量可以调控钯纳米颗粒的形貌．溶液中CTAB和还原剂浓度的改变,非常明显地影响合成产物中不同形貌钯纳米颗粒的产率．通过向溶液中引入柠檬酸离子调控纳米颗粒的成核与生长过程,首次合成出了星状钯二十面体和截面为五角星形的钯纳米棒．这些不同形貌的钯纳米颗粒有着不同的表面等离子体共振和表面增强拉曼散射性质．     

Colloidal AgCl induced-growth of thorny gold nanoparticles and their SERS activity

In this paper, we exploited a unique procedure for obtaining thorny gold nanoparticles (Au NPs) with controllable length of thorns without using seeds and surfactants. The obtained Au NPs exhibited shape-determined surface-enhanced Raman spectroscopy activity toward rhodamine 6G.     

Combined SPR and SERS microscopy in the Kretschmann configuration

A novel hybrid spectroscopic technique is proposed, combining surface plasmon resonance (SPR) with surface-enhanced Raman scattering (SERS) microscopy. A standard Raman microscope is modified to accommodate the excitation of surface plasmon-polaritons (SPPs) on flat metallic surfaces in the Kretschmann configuration, while retaining the capabilities of Raman microscopy. The excitation of SPPs is performed as in standard SPR-microscopy; namely, a beam with TM-polarization traverses off-axis a high numerical aperture oil immersion objective, illuminating at an angle the metallic film from the (glass) substrate side. The same objective is used to collect the full Kretschmann cone containing the SERS emission on the substrate side. The angular dispersion of the plasmon resonance is measured in reflectivity for different coupling conditions and, simultaneously, SERS spectra are recorded from Nile Blue (NB) molecules adsorbed onto the surface. A trade-off is identified between the conditions of optimum coupling to SPPs and the spot size (which is related to the spatial resolution). This technique opens new horizons for SERS microscopy with uniform enhancement on flat surfaces.     

Fabrication of nano-network gold films via anodization of gold electrode and their application in SERS

We report here a green and facile one-step method to fabricate nano-network gold films of low roughness via anodization of gold electrodes in an aqueous solution of l-ascorbic acid (AA) or hydroquinone (H₂Q) at the oxidation peak potential. The preparation involves the formation of thin gold oxide layer by anodization of gold and its simultaneous and/or subsequent reduction by AA or H₂Q. The as-fabricated nano-network gold films show very strong SERS activity in comparison with the substrates prepared by some other electrochemical roughening methods.     

Ultrathin Au nanowires and their transport properties

This paper presents a facile synthesis of single-crystalline Au nanowires by reduction of HAuCl4 in oleic acid and oleylamine. The diameter of these micron-meter-long Au nanowires is controlled to be 3 and 9 nm by volume ratio of oleylamine and oleic acid. When linked between two gold electrodes, the 9 nm Au nanowire shows good electron conductivity with its breakdown current density reaching 3.5 x 10(12) A/m2. This demonstrates that the chemically made ultrathin Au nanowires can be used as a molecular-scale interconnect for nanoelectronic applications.     

SERS spectroscopy and SERS imaging of Shewanella oneidensis using silver nanoparticles and nanowires

Facile and reproducible SERS signals from Shewanella oneidensis were obtained utilizing silver nanoparticles (AgNPs) and silver nanowires (AgNWs). Additionally, SERS images identify the distribution of SERS hot-spots. One important observation is the synergistically enhanced SERS signal when AgNPs and AgNWs are used in conjunction, due to constructively enhanced electromagnetic field.     

Surfactantless synthesis of multiple shapes of gold nanostructures and their shape-dependent SERS spectroscopy

A useful method for the synthesis of various gold nanostructures is presented. The results demonstrated that flowerlike nanoparticle arrays, nanowire networks, nanosheets, and nanoflowers were obtained on the solid substrate under different experimental conditions. In addition, surface-enhanced Raman scattering (SERS) spectra of 4-aminothiophenol (4-ATP) on the as-prepared gold nanostructures of various shapes were measured, and their shape-dependent properties were evaluated. The intensity of the SERS signal was the smallest for the gold nanosheets, and the flowerlike nanoparticle arrays gave the strongest SERS signals.     

One-step synthesis of gold nanoparticles using azacryptand and their applications in SERS and catalysis

A new aqueous-phase method for the preparation of stable gold nanoparticles by using 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (azacryptand) as both reductant and stabilizer is reported. Reduction of HAuCl(4) with azacryptand at room temperature yields nano-sized particles within a short time. The obtained gold nanoparticles have been characterized by UV-vis spectroscopy, transmission electron microscopy, and X-ray diffraction. Comparison of FT-IR spectra of azacryptand before and after reaction revealed that azacryptand molecules reduce gold ions as the amino moieties in the molecules are oxidized to imino groups. The prepared gold nanoparticles show efficient surface-enhanced Raman scattering properties and can effectively catalyze reduction of 4-nitrophenol by sodium borohydride in aqueous solution.     

Hydrothermal synthesis and their catalytic properties of manganite nanowires

La_0.5Sr_0.5MnO₃ nanowires and nanoparticles were synthesized by the hydrothermal and citrate methods, respectively. The samples were characterized by TEM, HRTEM, XRD, ICP and N₂ adsorption, etc. The formation mechanism of the nanowires was proposed and discussed. Further, the stabilities and catalytic activities of La_0.5Sr_0.5MnO₃ nanowires for CH₄ combustion were evaluated and compared with those of the nanoparticles. The results showed that the particle morphology had a significant effect on the properties of the catalysts. After running at a high temperature for a long duration, the nanowires showed a higher stability and a higher activity, compared with the nanoparticles. Their different stabilities were ascribed to the different surface energies (particle sizes), as well as to the different geometric packing models.     

Novel synthesis of organic nanowires and their optical properties

Aligned nanowires of organic luminescent material were prepared by introducing the organic luminants into nanochannels of variable size in an anodic aluminum oxide (AAO) membrane, and the emission spectra from these nanowire arrays exhibited novel size-dependent luminescent properties.     

SERS in PAH-Os and gold nanoparticle self-assembled multilayers

We present a detailed structural and surface-enhanced Raman scattering (SERS) study of poly(allylamine) modified with Os(byp)2ClPyCHO (PAH-Os) and gold nanoparticles self-assembled multilayers [PAH-Os+(Au-nanoparticlesPAH-Os)n, n=1 and 5]. Atomic force microscopy and variable-angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analyzing the sample thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The interaction between plasmons of near-gold nanoparticles provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single-plasmon resonance (approximately 530 nm). We compare the electronic resonance Raman and SERS amplification mechanisms in these self-assembled multilayers analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe large changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities which we relate to "hot spots." Numerical discrete dipole approximation calculations including the interaction between gold nanoparticles are presented, providing a qualitative model for the coupled-plasmon absorption and redshifted Raman hot spots in these samples.     

P-type 3C-SiC nanowires and their optical and electrical transport properties

We report for the first time the fabrication of p-type SiC nanowire field-effect transistors (FETs) using an individual Al-doped 3C-SiC nanowire with a single crystalline structure. The Raman spectroscopy of the as-grown p-type wire indicates that the linewidth and peak intensity of LO-phonon bands are sensitive to temperature variations.     

Structural and photocatalytic properties of silicon carbide powder and nanowires modified by gold nanoparticles

Research on Chemical Intermediates - Commercial bulk powder of SiC and SHS-synthesized SiC nanowires were studied. Gold nanoparticles were deposited onto a surface of both samples. Basic...     

Facile synthesis of gold nanoflowers as SERS substrates and their morphological transformation induced by iodide ions

We report a new strategy to prepare gold nanoflowers(AuNFs) using a two-step seed-mediated method. The as-prepared AuNFs were employed as surface-enhance Raman scattering(SERS) substrates, showing strong signal enhancement. We further found that iodide ions(I~–) could selectively induce the morphological transformation of AuNFs to spheres, resulting in a blue-shift of the localized surface plasmon resonance(LSPR) bands, a color change of the AuNFs solution from blue to red, and decreased SERS activity. This behavior allows the AuNFs to be used in the determination of I~–.     

电化学SPR协同催化对氯苯硫酚界面反应的SERS研究

表面等离激元共振(SPR)驱动的催化反应近年来广受关注, 研究集中在SPR等外场作用下发生单一的界面催化反应生成新的物质, 这对于表面反应设计和实现多步骤界面有机合成反应仍存在一定困难. 本工作以对氯苯硫酚(4-CBT)为探针, 利用表面增强拉曼光谱(SERS)的极高表面灵敏度并结合金粒子单层膜(Au MLF)的玻碳电极(Au MLF@GC电极)作为基底在均匀性方面的优势, 实现了电化学和SPR协同作用下界面催化反应及其过程的精准监测. 结果表明, 较负电位区间内, 在光电协同催化作用下, 4-CBT先发生脱氯反应生成苯硫酚(TP), 然后4-CBT脱氯与偶联同时发生生成联苯-4,4'-二硫醇(4,4'-BPDT). 提高激光功率可显著加快反应速率, 且在中性溶液中反应速率最快. 溴代苯硫酚以及邻位或间位二氯取代苯硫酚均可发生类似的脱卤素取代基和偶联反应, 但邻位二氯取代物因空间位阻而导致偶联效率降低.     

Zero-voltage conductance of short gold nanowires

Using the Landauer formula, the conductance of short gold wires is studied. The required electronic structure calculations are performed with a self-consistent tight-binding method. We consider gold wires of single-atom diameter with a variable number (N=1, em leader,5) of atoms. Depending on N, we find considerable conductance variations with one conductance quantum being the upper limit. The results are confirmed by means of Friedel's sum rule. Tip-shaped clusters are used to provide the contact-wire interfaces and the relation between various tip structures and the conductance is discussed. Our predictions about the conductance variations agree qualitatively with new experimental results.     

Synthesis, characterization and optical properties of silver and gold nanowires embedded in mesoporous MCM-41

Uniform nanowires of silver and gold inside the channels of MCM-41 were prepared by controlled reduction of their respective metal salts with sodium borohydride (NaBH₄). Presence of nanowires of silver and gold in MCM-41 were confirmed by high angle X-ray diffraction (XRD) data (peaks between 2ϑ = 30 − 60°) and transmission electron microscopy (TEM) confirmed the diameter of the nanowires. Diameter of nanowires is found to be ∼ 2.8 nm which is coincident with channel diameter of MCM-41. Optical properties of these heterostructured materials Ag-MCM-41 and Au-MCM-41 reveals the presence of surface plasmon absorption peaks of silver and gold respectively, and the shift in the absorption bands are associated to agglomeration of clusters inside the channels. Room temperature photoluminescence spectra exhibits interesting optical properties as observed for direct band gap semiconductors. Non-linear optical properties (NLO) corresponding to second harmonic generation (SHG) values were also recorded for self supported films of these heterostructured materials. Enhanced optical non-linearity was found to be arising from a corresponding increase of local field near the surface plasmon resonance. Further enhancement in SHG was found with poling due to an induction of orientation order.