Continuous Gold Films Research Articles

Multilayer Enhanced Gold Film over Nanostructure Surface-Enhanced Raman Substrates

We have developed a novel class of gold multilayer, surface-enhanced Raman scattering (SERS) substrates that are capable of enhancing SERS signals by 15.3-fold over conventional gold film over nanostructure (GFON) SERS substrates, making them comparable in sensitivity to optimized silver film over nanostructure (SFON) substrates, while providing the long-term stability obtained from gold. They are fabricated by depositing 10 A thick silver oxide islands on conventional GFON substrates, followed by deposition of a second continuous gold layer. The silver oxide layer acts as a dielectric spacer between the two continuous gold films and produces significantly enhanced SERS signals, as compared to optimized single layer substrates of the same geometry or comparable substrates prepared by deposition of silver islands that are not oxidized. In addition to the enhanced sensitivity of these multilayer substrates, they also exhibit long SERS active shelf-lives (i.e., months), with no measurable degradation in SERS enhancement, and relative standard deviations in SERS enhancement of less than 5.2% across the substrate's surface.

Surface plasmon coupled phosphorescence (SPCP)

We report the first observation of surface plasmon coupled phosphorescence (SPCP) for PtOEP (2,3,7,8,12,17,18-octaethyl-21H,23H-porphyrin platinum II) immobilized in polyvinyl chloride films on continuous gold films of 47 nm thickness. We confirm the observation of SPCP by comparing the spectra and lifetimes for plasmon coupled phosphorescence with the free space isotropic phosphorescence emission. Due to the directional emission inherent to plasmon coupled phosphorescence, we believe that SPCP can facilitate the study of phosphorescence in biological applications, such as protein dynamics and alkaline phosphatase studies, whose signal intensities are inherently weak and nearly always isotropic.

Deposition of densely packed gold film on an alkane derivative monolayer modified graphite surface

The growth of thin metal films is an important step in the fabrication of electronic and magnetic devices. In this work, an atomically flat graphite surface was used as a model system to understand the details of gold film growth mechanisms and kinetics. Ordered assembling monolayers of 1-octadecanethiol and stearic acid are used to modify the surface and uniform, densely packed ultrathin gold film with the thickness less than 5 nm are formed on these monolayer-modified graphite surfaces in a large area. The amount of gold needed to be deposited in order to form a continuous gold film is significantly reduced as compared to that needed on a bare graphite surface. Copyright © 2006 John Wiley & Sons, Ltd.

Formation mechanism of orderly structures in Au films deposited on silicone oil surfaces

An optical microscopy study of ordered structures, namely bands, and self-organized phenomena in a continuous gold film system deposited on silicone oil surfaces is presented. The bands are composed of a large number of parallel keys with different width w but nearly uniform length L; the characteristic length of the bands is of the order of 10 1–10 2 μm. After disturbed with an external force, the growth process of the bands is observed directly. The experiment indicates that the formation mechanism of bands can be explained in terms of the relaxation of the compressive stress, which mainly results from the characteristic boundary condition of the nearly free sustained films.

Microwave-assisted deposition of uniform thin gold film on glass surface

A microwave-assisted method for rapid deposition of continuous gold films on glass substrates has been developed. Under microwave irradiation, gold nanoparticles in colloidal solutions were shown able to assemble on the naked substrates to form a gold monolayer within minutes. Images obtained by AFM revealed that the monolayer formed on the naked glass surface was much the same as that formed on the glass substrates pre-silanized with (3-aminopropyl)-trimethoxysilane. With this monolayer as seeds, further deposition of gold metal on the surface was thus possible and the thickness of the deposited gold was time-dependent as in the case of normal electroless plating. The sum time for preparing a 50-nm gold film on glass was about 15 minutes, much shorter than the existed electroless methods. Electrochemical experiments showed that the prepared gold films were continuous and can be used as stable electrodes able to sustain several hundreds of redox circulation. The quality of the films were also verified by surface plasmon resonance imaging (SPRI) studies and quite sharp pictures of protein dots were obtained, showing a smooth gold surface and proper film thickness were prepared.

Ordered structures and self-organized phenomena in Au films deposited on silicone oil surfaces

An optical microscopy study of ordered structures and self-organized phenomena in a continuous gold film system deposited on silicone oil surface is presented. Band-shaped ordered patterns, namely bands, are formed after deposition under certain condition. The bands are composed of a large number of parallel key-shaped domains with different width but nearly uniform length. Further experiments show that the ordered patterns result from compressive stress in films. We also study the formation mechanism of internal stress in nearly free sustained films.

Fabrication of Au nanostructures in the process of amalgam formation followed by Au–Hg alloy thermal decomposition

Au films deposited under ultra-high vacuum conditions on air-cleaved mica substrates were exposed in situ to Hg vapor (residual gas pressure ∼1×10−7 Pa, with Hg vapor pressure of 0.24 Pa). This led to transformation of continuous gold film into isolated amalgam islands of nanometer scale. The changes of Au thin film surface topography caused by amalgamation carried out within ∼40 h were studied by the atomic force microscopy method, while the phase transition in the bulk of Hg-dosed Au films was monitored by means of X-ray diffraction. The island's morphology varies from irregular, ramified structures on terraces to compact shapes along the steps on the mica substrate. After thermal decomposition of the amalgam, thin gold films consisting of isolated Au nanostructures on mica can be obtained.

Ion-beam-induced embedded nanostructures and nanoscale mixing

Megaelectron volts ion-induced effects for discontinuous gold nanoislands and for continuous gold films on silicon substrate have been studied. Irradiation was carried out with 1.5MeV Au2+ ions at room temperature to various fluences. Cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry are used to study the ion-beam mixing in Au∕Si systems. At a fluence of 1×1014ionscm−2, a material push-in effect and a metastable Au-Si phase formation have been observed for Au nanoislands, while no push in or mixing has been observed for the case of continuous films. The mixed phase of Au-Si system is found to be crystalline in nature. The material push- in and ion-beam mixing effects that are observed in case of nanoislands appear to be due to combined effect of capillary driving force, ion-induced viscous flow, and ion-induced energy spike effects.

Reactive Scattering of CO from an Oxygen-Atom-Covered Au/TiO2 Model Catalyst

We present results of a reactive scattering study of CO from an oxygen-atom-covered Au/TiO2 model catalyst. We studied the CO oxidation reaction with preadsorbed atomic oxygen over a surface temperature range from 65 to 250 K and observed CO2 production at all temperatures studied from the gold-covered titania samples. No reaction is observed on the bare TiO2 (i.e., no gold) at any of the temperatures studied. The CO oxidation reaction is observed to proceed on all gold coverages studied (0.25−30 ML) as well as on a nominally continuous gold film. Hence, we conclude that the reaction of an oxygen atom with CO is relatively particle size independent and even independent of the TiO2 support and therefore not exclusive to the gold−titania interface, if oxygen atoms are preadsorbed prior to CO exposure.

Au/SiO2 Nanosystems by XPS

Au/SiO2 nanocomposites were prepared by rf-sputtering of gold on amorphous silica substrates. Deposition experiments were carried out in Ar plasmas at temperatures as low as 60 °C. Particular attention was devoted to the combined influence of the applied rf power and total pressure on the chemico-physical properties of the final nanosystems. In particular, low pressures (<0.2 mbar) and high rf powers (>20 W) resulted in a higher sputtering yield, allowing the deposition of continuous gold films on silica. Conversely, decreased sputtering yields (at higher pressures and lower powers) enabled the preparation of discontinuous Au/SiO2 nanosystems. A thorough investigation of the structure-properties relationships was attained by means of a multi-technique characterization. In particular, laser reflection interferometry (LRI) was employed for an in situ monitoring of growth processes, while glancing-incidence x-ray diffraction (GIXRD) and transmission electron microscopy (TEM) provided valuable information on the system nanostructure. Moreover, x-ray photoelectron spectroscopy (XPS), UV-Vis spectroscopy, and atomic force microscopy (AFM) were used to investigate the chemical composition, optical properties, and surface morphology, respectively. This study is dedicated to an XPS investigation of the principal core levels (Au, Si, O) of Au/SiO2 nanosystems. In particular, detailed scans for the Au 4f, Si 2s, O 1s, and C 1s regions and related data for a gold film on silica and a discontinuous Au/SiO2 specimen are presented and discussed.

Plastic-Compatible Low Resistance Printable Gold Nanoparticle Conductors for Flexible Electronics

Low resistance conductors are crucial for the development of ultra-low-cost electronic systems such as radio frequency identification tags. Low resistance conductors are required to enable the fabrication of high- inductors, capacitors, tuned circuits, and interconnects. The fabrication of these circuits by printing will enable a dramatic reduction in cost, through the elimination of lithography, vacuum processing, and the need for high-cost substrates. Solutions of organic-encapsulated gold nanoparticles many be printed and subsequently annealed to form low resistance conductor patterns. We describe a process to form the same, and discuss the optimization of the process to demonstrate plastic-compatible gold conductors for the first time. By optimizing both the size of the nanoparticle and the length of the alkanethiol encapsulant, it is possible to produce particles that anneal at low temperatures to form continuous gold films having low resistivity. We demonstrate the printing of these materials using an inkjet printer to demonstrate a plastic-compatible low resistance conductor technology. © 2003 The Electrochemical Society. All rights reserved.

New Strategy for Preparing Thin Gold Films on Modified Glass Surfaces by Electroless Deposition

This paper describes a new strategy for fabricating continuous gold films based on the self-assembly of the gold colloid monolayer on a poly(diallyldimethylammonium chloride)-modified glass slide, followed by electroless plating. Hydroxylamine-mediated reduction was proven as an excellent route to enlargement of immobilized nanoparticles on polymer-coated glass substrates in comparison to formaldehyde-mediated reduction. Au colloidal surface-catalyzed reduction of Au3+ by hydroxylamine exhibited very fast kinetics as monitored and confirmed by UV−vis spectroscopy in real time. The nanoscale morphology of the gold film was dependent on the initial coverage of gold nanoparticles and thermal annealing. Atomic force micrographs further revealed that enlarged particles were neither spherical nor cyclindrical, but highly complex in shape. The gold film thickness and its corresponding surface roughness could be easily controlled by setting the electroless deposition time. X-ray diffraction certified uniformity o...

Surface roughness and surface-induced resistivity of gold films on mica: Application of quantitative scanning tunneling microscopy

We report measurements of the resistivity $\ensuremath{\rho}(T)$ of a gold film 70 nm thick deposited on mica preheated to 300 \ifmmode^\circ\else\textdegree\fi{}C in UHV, performed between 4 and 300 K, and measurements of the surface topography of the same film performed with a scanning tunneling microscope (STM). From the roughness measured with the STM we determine the parameters \ensuremath{\delta} (rms amplitude) and \ensuremath{\xi} (lateral correlation length) corresponding to a Gaussian representation of the average height-height autocorrelation function (ACF). We use the parameters \ensuremath{\delta} and \ensuremath{\xi} to calculate the quantum reflectivity R and the increase in resistivity induced by electron-surface scattering on this film, according to a modified version of the theory of Sheng, Xing, and Wang (mSXW) [Munoz et al., J. Phys.: Condens. Matter 11, L299 (1999)]. The mSXW theory is able to select the appropriate scale of distance over which corrugations take place, leading to $R\ensuremath{\approx}1$ for corrugations taking place over scales of distances that are long when compared to a few Fermi wavelength ${\ensuremath{\lambda}}_{F},$ and $R<1$ for corrugations taking place over scales of distances that are comparable to ${\ensuremath{\lambda}}_{F}$ (to within an order of magnitude). The reflectivity R determined by corrugations ocurring over a scale of distances comparable to ${\ensuremath{\lambda}}_{F}$ approaches zero for a certain angle. The resistivity $\ensuremath{\rho}(T)$ of the film increases by roughly a factor of 4 between 4 and 300 K, and so does the bulk resistivity ${\ensuremath{\rho}}_{0}(T)$ predicted by mSXW theory. With the parameters \ensuremath{\delta} and \ensuremath{\xi} measured on our 70-nm film, we reproduced approximately the thickness and temperature dependence of the resistivity (between 4 and 300 K) of several gold films on mica reported by Sambles, Elsom, and Jarvis [Philos. Trans. R. Soc. London, Ser. A 304, 365 (1982)], without using any adjustable parameters. The results of this paper suggest that the relevant quantities controlling electron-surface scattering in continuous gold films of arbitrary thickness, are the parameters \ensuremath{\delta} and \ensuremath{\xi} describing the average ACF that characterizes the surface of the sample on a nanoscopic scale, in agreement with the accepted view regarding the conductivity of ultrathin films.

Molecular hydrogen interactions with discontinuous and continuous thin gold films

The influence of thin gold film surface nanostructure on H 2 chemisorption on unsintered gold films at 78 K was studied. Three classes of thin Au films of different thicknesses were chosen for these experiments, as follows: (i) very thin, purplish colour, transparent film formed by isolated Au islands, (ii) violet–green colour film formed by Au islands joined together and creating a kind of golden net, (iii) non-transparent, continuous Au film of golden colour. It has been found that H 2 chemisorption occurs only in the case when the thin Au films are deposited at low temperatures (78 K) and unsintered. It has been suggested that this chemisorption occurs on surface Au atoms of low coordination number as a result of formation of AuH 2 complexes, similarly to the compounds arising due to H 2 interaction with isolated Au atoms.

Electrochemical fabrication of n-Si/Au Schottky junctions

We report on the electrochemical deposition of gold films onto n-type silicon. Gold deposition occurs through progressive nucleation and diffusion limited growth. A high density of gold nuclei was obtained by using a short potential pulse to −1.6 V(Ag/AgCl), and subsequent growth was performed at about −1.1 V(Ag/AgCl) where the growth rate is kinetically limited. Transmission electron microscopy showed that high quality, continuous gold films were formed with an average grain size on the order of 50–70 nm. The electrical properties of the electrochemically deposited Si/Au Schottky junctions are comparable to junctions prepared by evaporation or sputtering techniques.

Determination of the surface area of annealed continuous gold film and pyrex glass in situ by the BET adsorption isotherm and ex situ by Atomic Force Microscopy in air

The relative surface area, Rsa, of annealed continuous thin gold film, thermally evaporated onto a Pyrex glass substrate at 78 K in an ultrahigh-vacuum (UHV) environment, and Pyrex glass has been determined in situ by the BET (Brunauer–Emmett–Teller) isotherm method, as well as ex situ by Atomic Force Microscopy (AFM) in air. It has been measured by the BET isotherm method that Rsa for clean, continuous and annealed gold film, investigated under UHV conditions, is equal to 6.3, whereas after its exposure to air, at about 100 Pa, Rsa decreased from 6.3 to 5.7. Further exposure to air, at atmospheric pressure, reduced Rsa from 5.7 to 2.2, whereas the exposure of contaminated Au film to the atomic hydrogen flux caused the increase of Rsa from 2.2 to 4.2. On the other hand, Rsa calculated directly from the AFM image of gold film in air is equal to 1.0. In contrast to thin gold film, the Pyrex glass surface area, measured under UHV conditions by the BET adsorption isotherm and in air by the AFM technique, is equal to its geometric area, i.e., Rsa=1.0.

Atomic force microscopy of [formula omitted] alloy formation on thin Au films

Atomic force microscopy was applied to the study of thin Au films deposited under ultra-high vacuum conditions on a glass support maintained at 78 K. A variety of thin gold film structures were thus determined and characterized ranging from (i) nanometric isolated gold islands, (ii) nanometric gold structures, to (iii) continuous gold films, all depending on the amount of Au deposited. The influence of the amalgamation process on the thin Au films topography was studied. The fundamental importance of surface defects on the rate of amalgamation was clearly seen. The movement of small Au islands on glass obtained as the result of thermal decomposition of Au Hg alloys was observed.

Diffusion of gold in 3C-SiC epitaxially grown on Si Structural characterization

The structural characteristics of gold/3C-SiC contacts, subjected to a long annealing at 500 °C for 500 h for aging investigation, were studied by transmission electron microscopy. After the aging test about 50% of the gold which was deposited on the SiC surface was diffused through the 2.5 μm thick 3C-SiC to the SiC/Si interface forming an almost continuous gold film resulting in a SiC/Au/Si structure. The intermediate gold film is in perfect epitaxial relation to the 3C-SiC overgrown and the Si substrate, in spite of the 25% misfit between the Au and Si. Moiré fringes produced by the interference of the Au and Si lattices reveal that the two lattices are perfectly relaxed. No linear or planar defects were observed in the gold film. No Au precipitates were observed inside the 3C-SiC overgrown or in the Si-substrate. A mechanism responsible for the formation of the intermediate gold film in the SiC/Si interface is proposed.

Atomic hydrogen adsorption on thin discontinuous and continuous gold films—similarities and differences

Atomic hydrogen adsorption on thin discontinuous Au films (nanostructures) and on continuous thin Au films was studied by means of TDMS (thermal desorption mass spectrometry). The surface topography of thin Au films was determined by means of AFM (atomic force microscopy). Thermal desorption (TD) spectra for hydrogen deposit similar for both cases were registered. However, at low coverage ( θ < 0.01), atomic hydrogen is somewhat more strongly bound on discontinuous nanostructured thin Au films.

Electrochemical Patterning of Self-Assembled Monolayers onto Microscopic Arrays of Gold Electrodes Fabricated by Laser Ablation

This report demonstrates proof-of-concept application of two established electrochemical techniques affecting self-assembled monolayersthe thermodynamic control of monolayer assembly at gold and the electrochemical desorption of self-assembled monolayers from goldto pattern monolayers onto gold microelectrodes. When combined with a simple laser-based microfabrication technique, using commercially available microscope adaptations, to form arrays of individually-addressable gold microelectrodes from continuous thin gold films on glass, two new patterning methodologies result that enable fabrication of monolayer-based affinity arrays for potential use in chemical and biosensors. The described microfabrication technique is central to the success of these patterning methodologies because it allows SAM patterning of metal arrays immediately after their fabrication, so as to avoid problems associated with forming SAMs on commercially-supplied arrays. In this paper, each patterning methodology is used to fabricat...