Charge‐transfer contributions in surface‐enhanced Raman scattering from Ag,Ag2S and Ag2Se substrates

The degree of charge‐transfer in Ag–4‐mercaptopyridine (Mpy) and Ag₂S–4‐Mpy systems is investigated by use of surface‐enhanced Raman spectroscopy (SERS). Ag₂S and Ag₂Se nanoparticles are prepared on the basis of the former formation of Ag nanoparticles to make the SERS analytical objects comparable. We utilize the intensity of the non‐totally symmetric modes (either b₁ or b₂) as compared with the totally symmetric a₁ modes to measure the degree of charge‐transfer. We find ~25% of charge‐transfer contribution for Ag–4‐Mpy, whereas 81 ~ 93% for Ag₂S–4‐Mpy. It means that the charge‐transfer resonance contribution dominates the overall enhancement in SERS of Ag₂S–4‐Mpy. Energy level diagram is applied to discuss the likely charge‐transfer transition between Ag, Ag₂S, Ag₂Se and 4‐Mpy. This article may point out the link among the three main resonance sources and could enable some insights into the electronic pathways available to the metal‐molecule and semiconductor‐molecule systems. Copyright © 2012 John Wiley & Sons, Ltd.     

银/二氧化钛核壳纳米颗粒对碲化镉纳米晶的荧光增强研究

利用新合成的复合纳米结构银/二氧化钛核壳纳米颗粒,研究了金属银纳米颗粒对碲化镉纳米晶层荧光的增强情况.结果表明,这种新型复合金属纳米结构能极大地增强发光纳米晶层的荧光强度.银/二氧化钛核壳纳米颗粒是以水合肼、硝酸银和四异丙氧基钛为原材料,利用胶体化学法在水溶液中合成.透射电子显微镜图片表明这种新合成的银/二氧化钛纳米材料基本上呈球形,有较为明显的核壳结构,中间黑色的核是银纳米颗粒,外层颜色较浅部分是二氧化钛壳层.另外,包裹二氧化钛壳层后,银纳米颗粒的表面等离子吸收带从409 nm红移至430 nm,也证实了这种新型核壳纳米材料的形成.将此合成方法得到的银/二氧化钛纳米颗粒和碲化镉纳米晶用旋转涂覆方法进行直接组合后,得到了银纳米颗粒对碲化镉纳米晶荧光的明显增强,并对其增强的物理过程进行了讨论.这种能够增强荧光团发光的新型复合银纳米结构将在发光器件、荧光成像、生物探测等方面具有一定的应用价值.     

Ag-Au合金/SiO$lt;sub$gt;2$lt;/sub$gt;复合薄膜的制备与表征

以AgNO₃,HAuCl₄和正硅酸乙酯为主要原料,利用溶胶-凝胶法和旋涂技术,通过热处理和紫外光辐射还原得到了不同n_Ag/n_Au（1∶0,2∶1,1∶2,0∶1）的Ag-Au合金/SiO₂复合薄膜.从扫描电子显微镜和X射线衍射谱的结果可以看出得到的薄膜均匀性好,复合薄膜中合金颗粒的尺寸为10 nm左右.利用紫外-可见分光光谱仪研究了复合薄膜的光吸收性能,结果表明,随着n_Ag/n_Au的降低,吸收峰的位置也由最初的Ag纳米粒子的等离子共振吸收峰430 nm附近,逐渐红移到Au纳米粒子的等离子共振吸收峰605和880 nm附近.从光吸收谱可以看出,n_Ag∶n_Au＝2∶1和1∶2的两个样品分别在515,730 nm附近和550,730 nm附近出现表面等离子共振吸收峰.这表明Au-Ag合金固溶体的形成. 关键词： 2')" href="#">Ag-Au合金/SiO₂ 紫外辐射 光吸收性能     

Ag<Subscript>2</Subscript>S/Ag heteronanostructure

An Ag₂S/Ag heteronanostructure has been prepared for the first time by hydrochemical deposition. The “acanthite α-Ag₂S–argentite β-Ag₂S” phase transformation has been studied in situ by high-temperature X-ray diffraction and transmission electron microscopy. The crystal structure of argentite has been revealed. It has been found that the concentration of vacant sites in the metal sublattice of argentite exceeds 92%. The reversible acanthite–argentite transformation in the Ag₂S/Ag heteronanostructure at the application of the external bias voltage is considered.     

Individual SERS substrate with core–satellite structure decorated in shrinkable hydrogel template for pesticide detection

Individual Au@PNIPAM/Ag composite has been designed and fabricated as surface‐enhanced Raman scattering (SERS) substrate in this paper. Because of the high porosity of the polymer shell and the driving force of the Au core to Ag⁺(H₂O)_n (n = 1–4) in aqueous solution, chemical reactions can be carried out while aggregation is completely avoided. Also, this makes the formation of vast and monodisperse Ag nanoparticles within PNIPAM and increases the colloidal stability. The Au cores with different sizes and the vast Ag nanoparticles then form core–satellite structures that can generate plasmon resonance. Moreover, this kind of individual Au@PNIPAM/Ag composite can be seen directly through Raman optical microscope, and uncertain effects on SERS signals resulting from variability of the configurations are minimized because these individual composite particles are relatively uniform. Importantly, the gaps between the Au and Ag nanoparticles can decrease because the PNIPAM shrinks from swollen to collapse state, so the substrate can also be used for inspecting pesticide residues accurately and rapidly. Copyright © 2014 John Wiley & Sons, Ltd.     

Corrosion processes of triangular silver nanoparticles compared to bulk silver

Excessive corrosion of silver nanoparticles is a significant impediment to their use in a variety of potential applications in the biosensing, plasmonic and antimicrobial fields. Here we examine the environmental degradation of triangular silver nanoparticles (AgNP) in laboratory air. In the early stages of corrosion, transmission electron microscopy shows that dissolution of the single-crystal, triangular, AgNP (side lengths 50–120 nm) is observed with the accompanying formation of smaller, polycrystalline Ag particles nearby. The new particles are then observed to corrode to Ag₂S and after 21 days nearly full corrosion has occurred, but some with minor Ag inclusions remaining. In contrast, a bulk Ag sheet, studied in cross section, showed an adherent corrosion layer of only around 20–50 nm in thickness after over a decade of being exposed to ambient air. The results have implications for antibacterial properties and ecotoxicology of AgNP during corrosion as the dissolution and reformation of Ag particles during corrosion will likely be accompanied by the release of Ag⁺ ions.     

Synthesis and formation mechanism of Ag–Ni alloy nanoparticles at room temperature

Ag–Ni nanoparticles were prepared with a chemical reduction method in the presence of polyvinylpyrrolidone (PVP) used as a stabilizing agent. During the synthesis of Ag–Ni nanoparticles, silver nitrate was used as the Ag⁺ source while nickel sulfate hexahydrate was used as Ni²⁺ source. Mixed solutions of Ag⁺ source and Ni²⁺ source were used as the precursors and sodium borohydride was used as the reducing agent. Five ratios of Ag⁺/Ni²⁺ (9:1, 3:1, 1:1, 1:3, and 1:9) suspensions were prepared in the corresponding precursors. Ag–Ni alloy nanoparticles were obtained with this method at room temperature. Scanning electronic microscope (SEM), energy dispersive spectrum (EDS), high resolution transmission electron microscope (HRTEM) were used to characterize the morphology, composition and crystal structure of the nanoparticles. The crystal structure was also investigated with X-ray diffraction (XRD). In all five Ag/Ni ratios, two kinds of particle structures were observed that are single crystal structure and five-fold twinned structure respectively. Free energy of nanoparticles with different crystal structures were calculated at each Ag/Ni ratio. Calculated results revealed that, with identical volume, free energy of single crystal particle is lower than multi-twinned particle and the difference becomes smaller with the increase of particle size; increase of Ni content will lead the increase of free energy for both structures. Formation of different crystal structures are decided by the structure of the original nuclei at the very early stage of the reduction process.     

AgI nanostructure development in sputter-disordered and Al-doped Ag films probed by XRD,SEM, optical absorption and photoluminescence

By use of thermal evaporation and rf magnetron sputtering 300 and 500 Å thick Ag and Ag (Al) films were prepared. γ-AgInanoparticles were formed during (a) short time (2–5 min) iodization of undoped thermally evaporated Ag films, (b) longer time (12 h) iodization of undoped rf sputtered Ag films and (c) short time (2–15 min) iodization of thermally evaporated Ag_0.95Al_0.05 and Ag_0.90Al_0.10 films of 500 and 300 Å thickness respectively. Both rf sputtered and Al doped Ag films yielded ～20 to ～60 nm sized γ-AgI particles upon iodization. Optical absorption spectra reveal Z_1,2 and Z₃ exciton transitions with increased broadening of γ-AgI nanoparticles, suggesting the effect of disorder produced during film formation. Blue shift observed with increasing film thickness could be the effect of decreasing particle size, thereby increasing the quantum confinement effects. Photoluminescence studies show that the donor-acceptor recombination rate, enhanced by 25% for Ag_0.95Al_0.05I film relative to that of undoped AgI, is due to the tight binding of Al to surface defect sites.     

Proof‐of‐Concept Studies Directed toward the Formation of Metallic Ag Nanostructures from Ag3PO4 Induced by Electron Beam and Femtosecond Laser

In this work, for the first time, the instantaneous nucleation and growth processes of Ag nanoparticles on Ag₃PO₄ mediated by femtosecond laser pulses are reported and analyzed. The investigated samples are pure Ag₃PO₄ sample, electron‐irradiated Ag₃PO₄ sample, and laser‐irradiated sample. Complete characterization of the samples is performed using X‐ray diffraction (XRD), Rietveld refinements, field emission scanning electron microscopy, and energy dispersive spectroscopy (EDS). XRD confirms that the irradiated surface layer remains crystalline, and according to EDS analysis, the surface particles are composed primarily of Ag nanoparticles. This method not only offers a one‐step route to synthesize Ag nanoparticles using laser‐assisted irradiation with particle size control, but also reports a complex process involving the formation and subsequent growth of Ag nanoparticles through an unexpected additive‐free in situ fabrication process.     

Phase transitions in CuS–Ag2S nanoparticle system

(Ag₂)_xCu_1?xS, x = .2, .4, .6 and .8 nanoparticles were synthesized by the solvothermal method. The as-synthesized nanoparticles were characterized by X-ray diffraction to study the crystal structure and size. The surface morphologies of the above samples were studied using scanning electron microscope. As there is continuous shift in the lower wavelength absorption edge of the UV–VIS spectrum of these samples with concentration, (Ag₂)_xCu_1?xS nanoparticles can be tuned to different band gap energies by varying the composition. The D.C. electrical resistance was measured in the temperature range 310–485 K. As Ag₂S transforms from monoclinic to bcc at around 450 K, copper sulfide nanoparticles also shows a phase transition at around 470 K, the effects of these two transitions are seen in the resistance measurements and in the UV–VIS spectra of the entire system. The electrical resistance of (Ag₂)_xCu_1?xS nanoparticles rapidly reduces as more and more copper sulfide is added.     

Effect of Coulomb blockade on the switching time of Ag–Ag2S–Pt atomic switches

A theoretical model based on the single electron tunneling phenomenon is employed to calculate the time-dependent electrical resistance of an Ag–Ag₂S–Pt atomic switch at different applied voltages. While a negative voltage is applied to Pt electrode, Ag atoms precipitate on the surface of Ag₂S electrode where they form Ag clusters. The resistance of switch decreases as Ag clusters grow larger between two electrodes. Our model calculations imply the time required to decrease the resistance of switch below the resistance quantum (switching time) is mainly determined by the Coulomb blockade effect of Ag clusters. The switching time is found to decrease exponentially with increasing the applied voltage, which agrees very well with the experimental observations.     

Features of the absorption spectra of thin films of M 2Ag1 − x CuxI3 solid solutions (M = Rb, Cs)

The absorption spectra of thin films of (MI)_{1 ? y} (Ag_{1 ? x} Cu_xI)_y solid solutions (M = Rb, Cs) with the initial molar concentration y = 0.33 have been investigated. It is established that, at low concentrations x, a local exciton band due to Cu⁺ ions is split off from the main long-wavelength exciton bands. In Rb₂Ag_{1 ? x} Cu_xI₃ solutions, the concentration shift of exciton bands indicates the formation of a persistent-type exciton spectrum. However, in Rb₂Ag_{1 ? x} Cu_xI₃ with x ≥ 0.5 and in Cs₂Ag_{1 ? x} Cu_xI₃ with x > 0.2, exciton spectra of amalgamation type are observed, which are related to the formation of more stable M ₃Ag_{2 ? 2x} Cu_2x I₅ solid solutions. The formation of these solutions leads to broadening of the exciton bands and to the concentration transition from persistent-to amalgamation-type exciton spectra.     

Fabrication of composite based on GeSi with Ag nanoparticles using ion implantation

Comparative analysis of the structural and optical properties of composite layers fabricated with the aid of implantation of single-crystalline silicon (c-Si) using Ge⁺ (40 keV/1 × 10¹⁷ ions/cm²) and Ag⁺ (30 keV/1.5 × 10¹⁷ ions/cm²) ions and sequential irradiation using Ge⁺ and Ag⁺ ions is presented. The implantation of the Ge⁺ ions leads to the formation of Ge: Si fine-grain amorphous surface layer with a thickness of 60 nm and a grain size of 20–40 nm. The implantation of c-Si using Ag⁺ ions results in the formation of submicron porous amorphous a-Si structure with a thickness of about 50 nm containing ion-synthesized Ag nanoparticles. The penetration of the Ag⁺ ions in the Ge: Si layer stimulates the formation of pores with Ag nanoparticles with more uniform size distribution. The reflection spectra of the implanted Ag: Si and Ag: GeSi layers exhibit a sharp decrease in the intensity in the UV (220–420 nm) spectral interval relative to the intensity of c-Si by more than 50% owing to the amorphization and structuring of surface. The formation of Ag nanoparticles in the implanted layers gives rise to a selective band of the plasmon resonance at a wavelength of about 820 nm in the optical spectra. Technological methods for fabrication of a composite based on GeSi with Ag nanoparticles are demonstrated in practice.     

A facile strategy to synthesize bimetallic Au/Ag nanocomposite film by layer-by-layer assembly technique

A facile strategy has been developed for the preparation of bimetallic gold–silver (Au–Ag) nanocomposite films by alternating absorption of poly-(ethyleneimine)–silver ions and Au onto substrates and subsequent reduction of the silver ions. The composition, micro-structure and properties of the {PEI–Ag/Au}_n nanocomposite films were characterized by ultraviolet visible spectroscopy (UV–vis), transmisson electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), surface enhanced Raman scattering (SERS) and cyclic voltammetry (CV). The UV–vis characteristic absorbances of {PEI–Ag/Au}_n nanocomposite thin film increase almost linear with the number of bilayers, which indicates a process of uniform assembling. Appearance of a double plasmon bands in the visible region and the lack of apparent core–shell structures in the TEM images confirm the formation of bimetallic Au–Ag nanoparticles. The result of XPS also demonstrates the existence of Ag and Au nanoparticles in the nanocomposite films. TEM and FESEM images show that these Ag and Au nanoparticles in the films possess sphere structure with the size of 20–25 nm. The resulting {PEI–Ag/Au}_n films inherit the properties from both the metal Ag and Au, which exhibits a unique performance in SERS and electrocatalytic activities to the oxidation of dopamine. As a result, the {PEI–Ag/Au}_n films are more attractive compared to {PEI–Ag/PSS}_n and {PEI/Au}_n films.     

A Combined Experimental and Theoretical Study on the Formation of Ag Filaments on β‐Ag2MoO4 Induced by Electron Irradiation

A combined experimental and theoretical study is presented to understand the novel observed nucleation and early evolution of Ag filaments on β‐Ag₂MoO₄ crystals, driven by an accelerated electron beam from an electronic microscope under high vacuum. The growth process, chemical composition, and the element distribution in these filaments are analyzed in depth at the nanoscale level using field‐emission scanning electron microscopy (FE‐SEM) and transmission electron microscopy (TEM) with energy‐dispersive spectroscopy (EDS) characterization. To complement experimental results, chemical stability, structural and electronic aspects have been studied systematically using first‐principles electronic structure theory within a quantum theory of atoms in molecules (QTAIM) framework. The Ag nucleation and formation on β‐Ag₂MoO₄ are a result of structural and electronic changes of the AgO₄ tetrahedral cluster as a constituent building block of β‐Ag₂MoO₄, consistent with Ag metallic formation. The formation of Ag filament transforms the β‐Ag₂MoO₄ semiconductor from n‐ to p‐type concomitant with the appearance of Ag defects.     

One-step, template-free route to silver porous hollow spheres and their optical property

Silver porous hollow spheres (SPHS) were fabricated via ultrasonic spray pyrolysis of aqueous solutions containing AgNO₃ and glucose. In the hot spherical liquid droplets, glucose, as reducing agent, reacted with Ag⁺ to form Ag nanoparticles, which subsequently moved to the periphery of the hot liquid droplets to form Ag nanoparticles-glucose hybrid shell. With the temperature further increased, aforementioned Ag nanoparticles melted to form Ag skeleton decorated with unreacted glucose, which converted to SPHS via dissolving unreacted glucose in water. Due to their porous hollow structures, SPHS exhibited a wide Vis-NIR adsorption in the range of 400-1100 nm.     

Co x Ag1−x core–shell nanoparticles: magnetic and magneto-optical studies

The magneto-optical properties of 14-nm Co _x Ag_1?x core–shell nanoparticles (x=0.7, 0.8, and 0.35) deposited on different substrates are investigated at room temperature in the photon-energy range from 0.8 to 4.8 eV. Particles with low Ag content show spectra very similar to pure Co nanoparticles while particles with high Ag content have totally different features, where the Ag plasma edge dominates the spectra. The spectral features of the polar Kerr rotation depend on particle composition. The ageing process and development of an oxide layer influence the particles’ core–shell structures and magnetization curves. Co-rich particles exhibit lower resistance to the oxidation process as compared to Ag-rich ones. The quality of the nanoparticles was checked by transmission electron microscopy in respect of time scale.     

Effect of surface charge on the colloidal stability and in vitro uptake of carboxymethyl dextran-coated iron oxide nanoparticles

An easy and novel routine are reported for the preparation of metallic silver nanoparticles (AgNPs) with controlled morphology, using Na⁺–magadiite swelled with hexadecyltrimethylammonium bromide (CTA⁺–magadiite) and a layered aluminophosphate with kanemite-type structure modified with n-dodecylammonium and n-butylammonium (but,dod-AlPO-kan) as hosts. For the preparation of the metallic AgNPs (Ag⁰) in the interlamellar space, the CTA⁺–magadiite and but,dod-AlPO-kan hosts were dispersed in N,N-dimethylformamide (DMF) solution with different AgNO₃ concentrations. DMF acts as reducing agent of Ag⁺ ions leading to nanoparticles with disk-like morphology of magadiite silicate; these were characterized by TEM and UV–Vis spectroscopy. On the other hand, the AgNPs are intercalated in but,dod-AlPO-kan showing spherical-like morphology. The UV–Vis spectra of the nanocomposites based on Ag⁰ and magadiite silicate show bands at 565 nm that can be attributed to Ag⁰ nanodisks. The Ag-but,dod-AlPO-kan-based nanocomposites present a band at 422 nm attributed to the surface plasmon resonance of Ag⁰ nanospheres. The results of transmission electron microscopy agree very well with XRD and UV–Vis analysis, indicating the formation of AgNPs with different morphologies using the two kinds of lamellar materials. The magadiite host has an important role in the synthesis of Ag nanodisks, because it controls the growth of nanoparticles inside the interlayer region with disk-like morphology due the high interlayer interactions of the silicate, leading to the growth of nanoparticles in only two directions (xy plane). On the other hand, when but,dod-AlPO-kan is used a sphere-like morphology is preferred due the best accommodation of AgNPs between the layers of aluminophosphate host.     

The thermoelectric power in AgxTiS2 and AgxNiPS3. Part II. The ionic component of the thermoelectric power

The EMF of the isothermal cells: Ag/AgI/Ag_xTiS₂: 0<x<1, T=150–200°C/Ag_xNiPS₃: 0<x<3, T=150–350°C has been measured. From the EMF-x curves the existence ranges of the 2-phase (stage I and II) regions ?0.16<x<0.32 for the Ag/Ag_xTiS₂ system at 190°C; 0.20 < x < 0.50 and 1 < x < 2 for the Ag/Ag_xNiPS₃ system at 400°C - have been determined. The results are sustained by X-ray diffraction and electrical conductivity measurements. From the EMF-T curves the partial enthalpy (ΔH?_Ag) and entropy (ΔS?_Ag) of dissolution of silver in the Ag_xSSE (solid solution electrode) materials were obtained. In the case of Ag_xTiS₂, ΔH?_Ag has a low absolute value, while ΔS?_Ag is distinctly positive. The EMF of the Ag/Ag_xNiPS₃ system also has a positive temperature coefficient. Furthermore, the ionic component of the thermoelectric power, ΔE/ΔT, of the thermogalvanic cells: Ag/AgI/Ag_xSSE/AgI/Ag Ag_xTiS₂: 0 < x < 1, T = 150–200°C( T ) (T+ΔT) Ag_xNiPS₃: 0 < x < 1, T= 150–350°C has been measured. The kinetically important heat of transport of silver ions in the Ag_xSSE materials has been determined in two ways: first from the dependence of the ionic Seebeck coefficient (?_Ag+) on reciprocal temperature; and second from direct calculation, using the data for ?_Ag+ and ΔS?_Ag. The heat of transport is much smaller than the activation enthalpy for Ag⁺-conduction, indicating a high ionic polaron binding energy in these materials.     

Highly Luminescent Material Based on Alq3:Ag Nanoparticles

Tris (8-hydroxyquinoline) aluminum (Alq₃) is an organic semiconductor molecule, widely used as an electron transport layer, light emitting layer in organic light-emitting diodes and a host for fluorescent and phosphorescent dyes. In this work thin films of pure and silver (Ag), cupper (Cu), terbium (Tb) doped Alq₃ nanoparticles were synthesized using the physical vapor condensation method. They were fabricated on glass substrates and characterized by X-ray diffraction, scanning electron microscope (SEM), energy dispersive spectroscopy, atomic force microscope (AFM), UV-visible absorption spectra and studied for their photoluminescence (PL) properties. SEM and AFM results show spherical nanoparticles with size around 70–80 nm. These nanoparticles have almost equal sizes and a homogeneous size distribution. The maximum absorption of Alq₃ nanoparticles is observed at 300 nm, while the surface plasmon resonant band of Ag doped sample appears at 450 nm. The PL emission spectra of Tb, Cu and Ag doped Alq₃ nanoparticles show a single broad band at around 515 nm, which is similar to that of the pure one, but with enhanced PL intensity. The sample doped with Ag at a concentration ratio of Alq₃:Ag?=?1:0.8 is found to have the highest PL intensity, which is around 2 times stronger than that of the pure one. This enhancement could be attributed to the surface plasmon resonance of Ag ions that might have increased the absorption and then the quantum yield. These remarkable result suggest that Alq₃ nanoparticles incorporated with Ag ions might be quite useful for future nano-optoelectronic devices.