Gold Island Films Research Articles

Periodic Nanotemplating by Selective Deposition of Electroless Gold Island Films on Particle-Lithographed Dimethyldichlorosilane Layers

Uniform hexagonal arrays of diverse nanotemplated metal structures were formed via selective electroless gold plating on particle-lithographed dimethyldichlorosilane layers. Surface-associated water at silica bead interstices was shown to correlate with the formation of silane rings with outer ring diameters ranging from 522.5+/-29.7 to 1116.9+/-52.6 nm and/or spherical gold nanoparticles with diameters from 145.5+/-20.2 to 389.1+/-51.1 nm in the array. Reproducibility and millimeter-size scalability of the array were achieved without the need for expensive and sophisticated lithography or metal deposition equipment. The formation of each structure was explained on the basis of the silanization mechanism and microscopic characterization, as well as dimensional analysis of the nanostructures. This new, facile, and versatile method enables fine fabrication of regular metal nanoparticle array platforms to improve optical and plasmonic features in nanoelectronics and nanophotonic devices.

A Raman Spectroscopic Investigation of Pyrite Oxidation and Flotation Reagent Interaction

The oxidation of pyrite and the interaction of xanthate or hydroxamate flotation collectors with the pyrite surface were investigated via normal and surface-enhanced Raman scattering (SERS) techniques. SERS was facilitated via vapour deposition of a discontinuous gold island film or through the application of silver colloids to the pyrite surface. The oxidation of pyrite produced a Raman-inactive solid phase, in addition to octasulfur. 442 nm irradiation could induce the formation of polymeric sulfur from the pyrite oxidation products. Dixanthogen was the major species present on pyrite conditioned with xanthate solutions under oxidizing conditions, although a bound xanthate species was also evident, particularly at reducing potentials. The interaction of hydroxamate with pyrite under oxidizing conditions produced a ferric hydroxamate compound.

Morphology and Refractive Index Sensitivity of Gold Island Films

The growing interest in recent years in gold island films prepared by vapor deposition on transparent substrates is largely attributed to the prominent localized surface plasmon (SP) extinction associated with nanostructured metal films. In the present study, two types of evaporated Au island films were investigated: (i) Au films (2.5, 5.0, and 7.5 nm nominal thickness) evaporated on silanized glass and annealed 20 h at a temperature <250 °C; (ii) Au films (7.5 and 10 nm nominal thickness) evaporated on unmodified glass and annealed 10 h at 550 or 600 °C. The 3D morphology of the Au islands was analyzed using high-resolution scanning electron microscopy (HRSEM), cross-sectional transmission electron microscopy (TEM), and atomic force microscopy (AFM) cross-sectional profilometry. Annealing at high temperatures, close to the glass transition temperature of the substrate, results in wetting of the Au islands by the glass and partial island embedding. The mechanism of morphology evolution during annealing ch...

Fractal structure of gold clusters formed under vacuum deposition on dielectric substrates

The growth of gold island films under vacuum deposition on the surface of dielectric substrates are investigated. Fractal dimensions of the clusters formed under film deposition at different substrate temperatures and evaporation rates are estimated. The correlation between surface morphology and optical properties of the films deposited are discussed.

Absorption spectroscopy of gold nanoisland films: optical and structural characterization

Nanoisland films prepared by annealing thin gold films at high temperatureswere imaged using scanning electron microscopy (SEM) and atomic forcemicroscopy, and optically characterized through absorption spectroscopy. Thingold films of effective thicknesses 2, 5 and 7 nm annealed at 500, 700 and900 °C were fabricated and studied experimentally. The measured absorption characteristics insupport of theoretical calculations showed that the shapes of gold islands werepartial spheres. The position of the peak absorption wavelength measured withs-polarized light or at normal incidence confirmed that the island shape grew from anear-hemisphere towards a sphere with increasing annealing temperature. The SEMimages confirmed that the size of islands increased from 15 nm in diameter to40 nm in diameter as film thickness increased from 2 to 5 nm. The affect of theindex of the substrate material on absorption characteristics were also studied bycomparing the absorption spectra of gold island films on quartz and LaSF15 glasssubstrates. The use of gold nanoisland films for preparing localized surface plasmonresonance substrates was suggested as they held advantages over the gold colloidfilms.

Electrical characterization of gold island films: A route to control of nanoparticle deposition

The evaporation of gold onto SiN or SiOx leads to the formation of nanoscale islands separated by tunneling barriers. As the deposited thickness of Au increases, there is a linear increase in island size and an exponential decrease in film resistance, resulting from coalescence of neighboring islands. This understanding of film morphology allows the construction of a simple model of film resistivity. The model also accounts for the exponential decrease of resistance observed during deposition of Pd nanoparticles onto Au island films. The results suggest the use of discontinuous Au island films as a sensitive way of controlling nanoparticle deposition.

Highly Stable Localized Plasmon Transducers Obtained by Thermal Embedding of Gold Island Films on Glass

Highly Stable Localized Plasmon Transducers Obtained by Thermal Embedding of Gold Island Films on Glass

Polymer-Coated Gold Island Films as Localized Plasmon Transducers for Gas Sensing

Ultrathin (typically < or = 10-nm thick) gold island films evaporated on transparent substrates show a prominent localized surface plasmon (SP) extinction in the visible-to-NIR range. Changes in the dielectric properties of the contacting medium influence the SP absorption band, providing a scheme for optical sensing based on refractive index change. In the present work, the gas sensing capability of gold island based localized surface plasmon resonance (LSPR) transducers was explored using polymeric coatings as the active interface. LSPR transducers were fabricated by spin-coating of polystyrene (PS) or polystyrene sulfonic acid, sodium salt (PSS) onto 5-nm-thick (nominal thickness) gold island films evaporated on silanized glass and annealed. Detailed characterization of the transducers was carried out using high-resolution scanning electron microscopy, cross-sectional transmission electron microscopy, and in situ atomic force microscopy under controlled atmosphere. The hydrophobic PS film exhibits swelling and significant thickness increase upon exposure to chloroform vapor and little or no change in water vapor, whereas the hydrophilic PSS film shows the opposite behavior when exposed to the same vapors. Polymer swelling upon absorption of vapors of good solvents shows a net effect of lowering the effective refractive index in the vicinity of the gold islands, manifested as a characteristic decrease of the SP band intensity and a blue shift of the band maximum. The response, measured for four different vapors, is fast (approximately 15 s) and reversible. It is shown that gold island systems coated with polymeric films can be applied to vapor recognition in an array configuration.

Biological Sensing and Interface Design in Gold Island Film Based Localized Plasmon Transducers

Discontinuous, island-type gold films (typically < or = 10 nm nominal thickness) prepared by evaporation of the metal on transparent substrates show a localized surface plasmon resonance (LSPR) extinction in the visible-to-NIR range and can be used as optical transducers for monitoring local refractive index change. Such transducers, operated in the transmission configuration, provide an effective scheme for label-free biological sensing using basic spectrophotometric equipment. Optimization of the sensitivity of LPSR transducers requires consideration of the distance between the metal island surface and the bound analyte, strongly affecting the optical response due to the fast decay of the evanescent field of localized plasmons. In the present work Au island based LSPR transducers were used to monitor antibody-antigen interactions, demonstrating the effect of the biorecognition interface thickness. Evaporated Au island films derivatized with IgG or hCG antigens were used as biological recognition elements for selective sensing of antibody binding, distinguishing between specific and nonspecific interactions. The LSPR results are supported by XPS and ellipsometry data as well as by AFM and HRSEM imaging, the latter providing actual visualization of the two protein binding steps. Increase of the recognition interface thickness leads to a concomitant decrease in the extinction and wavelength sensitivity, generally conforming to a model of an exponentially decaying surface plasmon (SP) evanescent field.

Transformed Gold Island Film Improves Light-to-Heat Transduction of Nanoparticles on Silica Capillaries

Gold (Au) nanoparticles (NP) assembled on inner walls of silica (SiO2) capillaries dissipated ≥10-fold more heat (≥96.92 ± 8.94 vs ≤9.92 ± 2.06%) from incident photons via localized surface plasmon resonance (LSPR) with a 10-fold faster response time (≤8.39 ± 1.05 vs ≥86.30 ± 8.61 s) than colloidal Au NPs suspended in H2O. Au NP assemblies were created by thermal transformation of gold island film (TGIF) that was electrolessly plated on the inner capillary wall to form an optical plasmon capillary (OPC). Enhanced laser-to-heat transduction of the solid-state Au NP OPC resulted from the ability to tune the LSPR peak to the laser line, elimination of laser- or heat-induced NP aggregation, and enhanced thermal transport of the Au NP assembly on the OPC. This yielded ≥6.4-fold higher temperature increase per unit incident laser power (157.07 ± 21.68 vs 24.51 ± 4.45 °C watt−1) relative to suspended Au NPs. Thermal behavior of the OPC was predictable using design equations obtained from an overall energy balanc...

Electrical conductivity and luminescence of metal-organic nanocomposites

We presented the results of experimental study of electrical and luminescence properties of planar nanocomposites based on gold island films and layers of efficient organic luminophores — aluminum (III) 8-hydroxyquinolinate (Alq3) and europium β-diketonates [Eu(DBM)3 bath, Eu(DBM)3 phen, and Eu(DBM)3]. The mechanisms of electroluminescence of such hybrid systems are discussed and the processes of luminescence excitation in the organic component of nanocomposites are considered in detail. It is shown that the spectral characteristics of europium β-diketonates are controlled by the Eu3+ ion fluorescence. The proposed mechanisms of nanocomposite electroluminescence are confirmed by the results of measurements of cathodoluminescence spectra of organic luminophores.

Epitaxial Growth of Aligned Semiconductor Nanowire Metamaterials for Photonic Applications

AbstractA novel class of optical metamaterials is presented consisting of high densities of aligned gallium phosphide (GaP) nanowires fabricated using metal‐organic vapor phase‐epitaxy. Starting from a gold island film as a catalyst for nanowire growth, a sequential combination of vapor–liquid–solid and lateral growth modes is employed to obtain a continuous tunability of the nanowire volume fraction from 7% to over 35%. By choosing different crystallographic orientations of the GaP substrate, metamaterials are designed with different nanowire orientations. The anisotropy of the nanowire building blocks results in strong optical birefringence. Polarization interferometry demonstrates a very large polarization extinction contrast of 4 × 103 combined with a sharp angular resonance which holds promise for optical sensing. Nanowire metamaterials may find applications in photonics, optoelectronics, non‐linear and quantum optics, microfluidics, bio‐, and gas sensing.

Gold island films on indium tin oxide for localized surface plasmon sensing

Mechanically, chemically and optically stable gold island films were prepared on indium tinoxide (ITO) substrates by direct thermal evaporation of thin gold films (2–6 nm)without the need for pre- or post-coating. The effect of mild thermal annealing(150 °C, 12 h) or short hightemperature annealing (500 °C, 1 min) on the morphology of the gold nanostructures wasinvestigated. ITO covered with 2 nm gold nanoislands and annealed at500 °C for 1 min was investigated for its ability to detect the adsorption of biotinylated bovineserum albumin using local surface plasmon resonance (LSPR), and its subsequentmolecular recognition of avidin.

Electroless Gold Island Thin Films: Photoluminescence and Thermal Transformation to Nanoparticle Ensembles

Electroless gold island thin films are formed by galvanic replacement of silver reduced onto a tin-sensitized silica surface. A novel approach to create nanoparticle ensembles with tunable particle dimensions, densities, and distributions by thermal transformation of these electroless gold island thin films is presented. Deposition time is adjusted to produce monomodal ensembles of nanoparticles from 9.5 +/- 4.0 to 266 +/- 22 nm at densities from 2.6 x 1011 to 4.3 x 108 particles cm-2. Scanning electron microscopy and atomic force microscopy reveal electroless gold island film structures as well as nanoparticle dimensions, densities, and distributions obtained by watershed analysis. Transmission UV-vis spectroscopy reveals photoluminescent features that suggest ultrathin EL films may be smoother than sputtered Au films. X-ray diffraction shows Au films have predominantly (111) orientation.

Use of aluminum films as substrates for enhanced fluorescence in the ultraviolet-blue spectral region.

Thermal evaporation was used to deposit particulate aluminum films of varied thicknesses on quartz substrates. These substrates were characterized by scanning electron microscopy (SEM), which reveal that with an increase in aluminum thickness, the films progress from particulate towards smooth surfaces. Until now, metal-enhanced fluorescence (MEF) has primarily been observed in the visible-NIR wavelength region using silver or gold island films and roughened surfaces. We now report that fluorescence can also be enhanced in the ultraviolet-blue region of the spectrum using nano-structured aluminum films. We used two probes, one in the ultraviolet (a DNA base analogue 2-aminopurine: 2-AP) and another one in blue spectral region (a coumarin derivative: 7-HC) for the present study. We observed increased emission, decrease in fluorescence lifetime and increase in photostability of the dyes in a 10 nm spin-casted polyvinyl alcohol film on the Al nanostructured surfaces. We observe that the fluorescence enhancement factor depends on the thickness of the Al films because the size of the nanostructures formed varies with Al thickness. These studies indicate that Al nano-structured substrates can potentially find widespread use in MEF applications particularly in the UV - blue spectral regime. Finite-Difference Time-Domain (FDTD) calculations were performed that revealed enhanced near-fields induced around aluminum nanoparticles by a radiating fluorophore emitting at the emission wavelength of 2-AP. The effect of such enhanced fields on the fluorescence enhancement observed is also discussed.

Aluminum nanostructured films as substrates for enhanced fluorescence in the ultraviolet-blue spectral region.

Particulate aluminum films of varied thicknesses were deposited on quartz substrates by thermal evaporation. These nanostructured substrates were characterized by scanning electron microscopy (SEM). With the increase of aluminum thickness, the films progress from articulate toward smooth surfaces as observed by SEM images. To date, metal-enhanced fluorescence (MEF) has primarily been observed in the visible - NIR wavelength region using silver or gold island films or roughened surfaces. We now show that fluorescence could also be enhanced in the ultraviolet-blue region of the spectrum using nanostructured aluminum films. Two probes, one in the ultraviolet and another one in the blue spectral region, have been chosen for the present study. We observed increased emission, decrease in fluorescence lifetime, and increase in photostability of a DNA base analogue 2-aminopurine and a coumarin derivative (7-HC) in 10-nm spin-casted poly(vinyl alcohol) film on Al nanostructured surfaces. The fluorescence enhancement factor depends on the thickness of the Al films as the size of the nanostructures formed varies with Al thickness. Both probes showed increased photostability near aluminum nanostructured substrates, which is consistent with the shorter lifetime. Our preliminary studies indicate that Al nanostructured substrates can potentially find widespread use in MEF applications particularly in the UV-blue spectral regime. Furthermore, these Al nanostructured substrates are very stable in buffers that contain chloride salts compared to usual silver colloid-based substrates for MEF, thus furthering the usefulness of these Al-based substrates in many biological assays where high concentration of salts are required. Finite-Difference Time-Domain calculations were also employed to study the enhanced near-fields induced around aluminum nanoparticles by a radiating fluorophore, and the effect of such enhanced fields on the fluorescence enhancement observed was discussed.

Electrodeposition of gold nanoparticles on fluorine-doped tin oxide: Control of particle density and size distribution

Electrodeposition of gold island films of nanometre dimensions (<100 nm) on fluorine-doped tin oxide (FDTO) coated glass is reported. The effect of altering the overpotential of the potentiostatic transient, the use of both nucleation and growth pulses as well as the immobilisation of a spontaneously adsorbed monolayer of, 3-aminopropyldimethylmethoxysilane, is reported. Deposits ranging from isolated particles to dense arrays of nanometre dimension particles have been formed. For example, by using a 10 ms nucleation pulse at -1200 mV followed by growth at 600 mV, a high particle density (46 particles/μm 2 ) and a mean particle size of 36 nm with an RSD of 29% were obtained. Double potential step approaches coupled with interfacial engineering via monolayer formation allows gold nanoparticles to be created on optically transparent FDTO with controlled particle size, density and particle size distribution in a facile, inexpensive, reproducible manner.

Attenuated Total Reflection Surface-Enhanced Infrared Absorption Microspectroscopy for Identification of Small Thin Layers on Material Surfaces

Attenuated total reflection surface-enhanced infrared absorption microspectroscopy (micro-ATR-SEIRA) was developed for the identification of sub-mm size and nm-thick layers on material surfaces by using gold island films deposited on the surface of micro-ATR crystals. A thin layer of triphenyl phosphate (TPP) on a poly(tetrafluoroethylene) (PTFE) membrane filter was used to evaluate the enhancement of the absorption bands. Three types of crystals: diamond, silicon, and germanium, were evaluated. Diamond gave the greatest enhancement with a 12 nm thick gold island film. The enhancement factor was 200 times compared to bare diamond crystal, whereas it was 10 times for germanium crystal. This variation of enhancement factor according to crystal types was presumed to be due to the morphology of the gold films on the crystals. We also obtained an enhanced ATR map over an area of approximately 2 x 6 mm for a thin layer (approximately 1 nm thick) of di-2-ethylhexylphthalate on PTFE using gold-coated hexagonal silicon micro-ATR crystals. This micro-ATR-SEIRA technique has major potential for analyzing small and thin substances on material surfaces.

Silica-Stabilized Gold Island Films for Transmission Localized Surface Plasmon Sensing

Ultrathin gold films prepared by evaporation of sub-percolation layers (typically up to 10 nm nominal thickness) onto transparent substrates form arrays of well-defined metal islands. Such films display a characteristic surface plasmon (SP) absorption band, conveniently measured by transmission spectroscopy. The SP band intensity and position are sensitive to the film morphology (island shape and inter-island separation) and the effective dielectric constant of the surrounding medium. The latter has been exploited for chemical and biological sensing in the transmission localized surface plasmon resonance (T-LSPR) mode. A major concern in the development of T-LSPR sensors based on Au island films is instability, manifested as change in the SP absorbance following immersion in organic solvents and aqueous solutions. The latter may present a problem in the use of Au island-based transducers for biological sensing, usually carried out in aqueous media. Here, we describe a facile method for stabilizing Au island films while maintaining a high sensitivity of the SP absorbance to analyte binding. Stabilization is achieved by coating the Au islands with an ultrathin silica layer, ca. 1.5 nm thick, deposited by a sol-gel procedure on an intermediate mercaptosilane monolayer. The silica coating is prepared using a modified literature procedure, where a change in the reaction conditions from room temperature to 90 degrees C shortened the deposition time from days to hours. The system was characterized by UV-vis spectroscopy, ellipsometry, XPS, HRSEM, AFM, and cyclic voltammetry. The ultrathin silica coating stabilizes the optical properties of the Au island films toward immersion in water, phosphate buffer saline (PBS), and various organic solvents, thus providing proper conditions where the optical response is sensitive only to changes in the effective dielectric constant of the immediate environment. The silica layer is thin enough to afford high T-LSPR sensitivity, while the hydroxyl groups on its surface enable chemical modification for binding of receptor molecules. The use of silica-encapsulated Au island films as a stable and effective platform for T-LSPR sensing is demonstrated.

Giant nonlinear refraction in gold island films

Giant nonlinear refraction has been experimentally observed in gold island films. The real part of the third-order nonlinear susceptibility χ(3)(ω: ω,-ω, ω) reaches a value of about 10−4 CGS units. The mechanism of nonlinearity of the refractive index can be associated with the resonance enhancement of the factor of the local optical field at the pump wavelength due to the excitation of surface plasmons, as well as with the contribution of the heating of conduction electrons in an ensemble of metal particles.