Surface enhanced infrared absorption observed with attenuated total reflection (ATR-SEIRA): modeling the optical response

The gain in sensitivity due to Surface Enhanced Infrared Absorption (SEIRA) when analyzing an adsorbate on a metal island film was calculated for the ATR configuration under full consideration of the anisotropy of the film. Modeling the islands as spheroids coated with the dielectric adsorbate leads to such extraordinarily high refractive indices and low absorption indices as experimentally observed and to an enhancement of the absorption bands of the coating. Simulation of the ATR reflectance indicated further enhancement as a result of the optical properties of the island film. The predicted total level of enhancement was about one order of magnitude superior to what commonly is found in experiments. This was attributed to percolation of the metal islands which almost inevitably occurred in practice: the resulting lateral conductivity was found to have a strongly decreasing effect on the enhancement. Received: 23 December 1997 / Revised: 9 March 1998 / Accepted: 15 March 1998     

Surface-enhanced ATR-IR spectroscopy with interface-grown plasmonic gold-island films near the percolation threshold

Flat nano-island films prepared by wet-chemical deposition were investigated with attenuated total reflection infrared (ATR-IR) spectroscopy and scanning electron microscopy (SEM) in order to analyze the correlation between film morphology and optical properties. Here we choose Au as representative coinage metal (Au, Ag, Cu) that shows strong structure-dependent surface-enhanced infrared absorption (SEIRA). Infrared spectra of octadecanethiol monolayers on films of different stages of morphologic development show effects that are characteristic for SEIRA, such as enhanced vibrational signals, Fano-type line shape, and adsorbate induced baseline shifts. Their extent was found to be strongly dependent on the structural details and the strongest enhancement occurs at the percolation threshold of the two-dimensional island system. Also films beyond percolation show significant enhancement due to residual nanoholes that are acting as hotspots.     

ATR FT-IR absorption enhancement of a thin film under the photon-tunneling condition.

A simple, yet very powerful technique for the spectral acquisition of an extremely thin film with enhanced absorption was explored. An infrared absorption of an extremely thin film confined between media of high refractive indices was greater than that of its bulk when the spectrum was acquired under the attenuated total reflection (ATR) condition with parallel (p) polarized radiation. The absorption enhancement was not observed under perpendicular (s) polarized radiation. Theoretical investigations indicated that the absorption enhancement was proportional to the integration of the mean square evanescent field within the film. The field integration under p-polarized radiation increased, while that under s-polarized radiation decreased as the thickness of the confined film became thinner. The maximum enhancement was observed when the film was sufficiently thinner than the penetration depth. The phenomena were experimentally investigated, and the results agreed very well with theoretical predictions.     

钴岛膜上对巯基苯甲酸的表面增强红外吸收光谱

通过置换反应在金属铝表面制备了表面没有任何保护剂且具有红外增强作用的钴岛膜,用SEM、XRD和表面增强红外光谱对其形貌和性质进行表征。 结果表明,铝片上沉积出的钴呈岛状结构,钴岛膜由二次钴粒子和一次钴粒子通过密堆积的方式构成;首次发现具有这种特殊结构的钴对吸附于其表面的有机分子的红外吸收光谱有较大的增强作用,用此钴岛膜对1 mmol/L的对巯基苯甲酸的红外光谱研究时得到很好的红外增强信号,使得表面增强红外光谱可以用于痕量分析、检测。     

Influence of local structure on local electric field in Island‐like silver films

The infrared absorption enhancement phenomenon in the normal configuration of vacuum‐evaporated metal films on a transparent substrate is known to depend not only on the metal film morphology but also on the local structures of metal particles. To date, however, few studies have examined the effect of local structure on the phenomenon. Size distributions of islands and gaps, along with the volume fractions of Ag in thin films, were measured using scanning electron microscopy as a function of film thickness. The local structure of Ag nano clusters deposited on silicon substrates was investigated using a total conversion electron yield X‐ray absorption fine structure (XAFS) method at the Ag K‐edge. We observed a correlation between the electromagnetic field intensity at the surface as evaluated by IR measurement and the coordination numbers evaluated by XAFS. We found that the film morphology had a greater effect on resonant and nonresonant contributions than did the local structure of a particle. Copyright © 2006 John Wiley & Sons, Ltd.     

Surface enhanced fluorescence and Raman imaging of Langmuir-Blodgett azopolymer films

The spectroscopic properties and surface-enhanced spectra of Langmuir-Blodgett (LB) films of methacrylic homopolymer (HPDR13) are presented. It is shown that LB film displays strong fluorescence attributed to the spatial restrictions imposed by its structure. The emission is observed in conjunction with photoisomerization, a process clearly demonstrated by the formation of surface-relief gratings in the LB film [C.R. Mendon?a et al., Macromolecules 32 (1999) 1493]. Surface-enhanced Raman scattering (SERS), Surface-enhanced resonance Raman scattering (SERRS) and surface-enhanced fluorescence (SEF) were observed for LB films of HPDR13 deposited onto silver island films. SERS measurements were also carried out on a sample fabricated with one monolayer LB film deposited onto silver islands followed by one overlayer of silver (LB sandwiched between two layers of silver islands). The polymer interacts very weakly with the metal surface (physisorption), and the enhancement effect is determined by the local electric field enhancement. The strong SERS and SERRS signals were suitable for micro-Raman imaging. Line, area mapping and global images of the LB monolayer on silver island are reported. The transfer ratio in the fabrication of the LB suggests a homogeneous coating of the silver islands, thereby the chemical images show the variation of the SERS intensity due to surface enhancement.     

Dielectric interlayers for increasing the transparency of metal films for mid-infrared attenuated total reflection spectroscopy

By depositing a continuous, thin metal film on a substrate coated with a mid-infrared (IR) transparent dielectric film that fulfils the role of an index-matching, anti-reflective coating for the metal, the transparency of the metal in the IR wavelength range can be significantly enhanced. This effect is used to yield enhanced absorption in attenuated total internal reflection infrared (ATR-IR) spectroscopy in the presence of continuous thin metal films. The main limitation of the ATR technique when using continuous metal films is the low transparency of metals, especially for infrared light. Computations and experiments show an enhancement in the absorbance of a sample in contact with the metal at certain wavenumbers when the dielectric interlayer is present. The realisation of the setup is the stratified system consisting of zinc selenide-germanium (~1 μm)-gold (40 nm and 20 nm) using the organic solvent acetonitrile as sample. Enhancement is stronger in s- than in p-polarisation. In s-polarisation, enhancement factors of up to 4 have been observed so far in experiments, but calculations show a route to higher enhancements. In addition to the increased absorption, the absorbance spectra show interference fringes which are due to a mismatch in the real part of the refractive index of the sample in contact with the metal film compared to a reference measurement.     

Induced surface enhancement in coral Pt island films attached to nanostructured Ag electrodes

Coral Pt islands films are deposited via electrochemical reduction on silica-coated nanostructured Ag electrodes. From these devices surface-enhanced (resonance) Raman [SE(R)R] signals of molecules exclusively attached to Pt are obtained with intensity up to 50% of the value determined for Ag. SE(R)R spectroscopic investigations are carried out with different probe molecules, silica-coating thicknesses, and excitation lines. Additionally, field enhancement calculations on Ag-SiO(2)-Pt support geometries are performed to elucidate the influence of the Pt island film nanostructure on the observed Raman intensities. It is concluded that the nonperfect coating of the Pt island film promotes the efficiency of the induced Pt SER activity. Comparison with similar measurements on Ag-SiO(2)-Au electrodes further suggests that the chemical nature of the deposited metal island film plays a minor role for the SE(R)R intensity.     

Enhanced Fluorescence Using an Optical Interference Mirror Overlaid with Silver Island Film

Fluorophores overlaid on an optical interference mirror composed of a metal and thin dielectric layer demonstrate enhanced fluorescence. Fluorescence is also enhanced by silver nanostructures such as silver island films, which excite localized surface plasmon resonance. An optical interference mirror surface was overlaid with a silver island film to amplify the fluorescence enhancement. The optimal thickness of the silver island film (100 nm) was evaluated from transmittance and surface roughness measurements. At this thickness, the fluorescence was amplified sixteen-fold. The thickness of the interference layer was optimized at 40 nm providing a one hundred-sixty fold fluorescence enhancement of rhodamine B. However, only a four-fold improvement in sensitivity was achieved for the determination of a labeled streptavidin using biotin immobilized on the silver island film interference mirror.     

Silver Nano Islands Enhanced Raman Scattering on Large Area Grating Substrates Fabricated by Two Beam Laser Interference

The authors preparedlarge area surface-enhanced Raman scattering(SERS) active substrates with tunable enhancement. First the large area gratings were fabricated by scanning a photoresist with two-beam laser interference and subsequently they were coated with silver nano islands via vacuum evaporation. SERS active metal island grating substrates with four different periods(300, 400, 515 and 600 nm) and Ag nano islands uniformly coated on an area of 2.5 cm×0.5 cm were obtained. The measured SERS spectra reveal the tuning effect of the period on the Raman signals period. The highest enhancement(ca. 10⁵) for Rhodamine 6G(R6G) as probing molecule is associated with a period of 515 nm due to the perfect matching of surface plasmons and Raman excitation line. A good reproducibility of SERS signals with almost the same SERS intensity at different spots was observed on all the larger area Ag island grating substrates.     

Formation of Ultrathin Films at the Solid-Liquid Interface Studied by In Situ Ellipsometry

Ellipsometric investigations of self-assembled monolayers (SAMs) of alkylsiloxanes on native silicon substrates and of organothiols on gold substrates were performed under in situ conditions with the substrate in direct contact with the adsorbate solution. Specially designed liquid cells matched for different incidence angles were used to carry out measurements in a range of organic solvents with different refractive indices as the ambient medium. The observed shifts in the ellipsometric phase angles Delta upon monolayer formation were found to depend very sensitively on the incidence angle and the refractive indices of the adsorbate film and the ambient solvent, from which a rather simple method for determining the refractive index of the adsorbate film, based on a variation of the ambient refractive index, was derived. Time-resolved in situ measurements of SAM formation in different solvents and onto different substrates yielded accurate kinetic information on the monolayer growth process and revealed hitherto unknown strong solvent effects on the growth rate. Copyright 1999 Academic Press.     

Langmuir-Blodgett monolayers of long-chain NBD derivatives on silver island films: Well-organized probe layer for the metal-enhanced fluorescence studies

The metal-enhanced fluorescence (MEF) from the well-organized monolayers of two newly prepared long-chain alkylamine derivatives of nitrobenzoxadiazole (NBD-C16 and NBD-C18) deposited on silver island films (SIFs) has been investigated. The NBD derivatives were conveniently prepared by using a single step procedure in quantitative yields. The monolayers of the probes on SIFs as well as on bare-glass slides were obtained by using the Langmuir-Blodgett (LB) technique. Orientation of the NBD probe molecule in LB monolayer film was measured with polarized absorption spectroscopy. The observed tilt angle of the probe transition dipole moment with respect to the surface normal of approximately 67-68 degrees was evaluated. We observed that the NBD monolayers deposited in close proximity to silver islands show about a 16-fold increase in fluorescence intensity and shortened fluorescence lifetime compared to those on bare-glass, which are due to metal enhanced fluorescence. On the other hand, the corresponding MEF from randomly oriented film obtained by using spin coating of the probes on SIFs was only 2.5-fold. Further, we deposited mixed monolayers of NBD-C16 or NBD-C18 with various molar ratios of stearic acid to understand the polarity effect on MEF. Interestingly, we found a consistent increase in MEF efficiency with increasing molar ratio of stearic acid. Along with MEF, we also found a continuous blue-shift in emission band maxima of the probes with an increasing molar ratio of stearic acid. The observed increase in MEF efficiency is justified based on cooperative effects of (1) the modulations in electronic density of the surface plasmon absorption band of SIFs and (2) defined probe orientation that might lead to preferential excitation.     

Electroless deposition of gold thin films on silicon for surface-enhanced infrared spectroelectrochemistry

Electroless (or chemical) deposition technique has been used in preparing Au island film electrodes on Si for in situ infrared spectroscopic studies of the electrochemical interface in attenuated total reflection mode. Owing to surface-enhanced infrared absorption (SEIRA) effect, absorption bands of molecules adsorbed on the chemically deposited films were one order of magnitude as large as those observed on smooth Au electrode surfaces. Although the enhancement factor was identical to that observed on vacuum evaporated Au island films, this simple method is superior to vacuum evaporation method with respect to the adhesion of the film, surface contamination, reproducibility, and cost.     

Biological sensing using transmission surface plasmon resonance spectroscopy

Ultrathin gold island films evaporated on transparent substrates offer promising transducers for chemical and biological sensing in the transmission surface plasmon resonance (T-SPR) mode. In the present work, the applicability of T-SPR-based systems to biosensing is demonstrated, using a well-established biological model system. Au island films were evaporated on polystyrene slides and modified with a biotinylated monolayer via a multistep surface reaction, the latter assisted by the good adhesion of metal islands to polystyrene. The biotin-derivatized Au island film was then used as a biological recognition surface for selective sensing of avidin binding, distinguishing between specific and nonspecific binding to the substrate. Transduction of the binding event into an optical signal was achieved by T-SPR spectroscopy, using plasmon intensity measurements, rather than wavelength change, for maximal sensitivity and convenience. T-SPR spectroscopy of Au island films is shown to be an effective tool for monitoring the binding of biological molecules to receptor layers on the Au surface and a promising approach to label-free optical biosensing.     

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.     

Interaction of CO and deuterium with bimetallic, monolayer Pt-island/film covered Ru(0001) surfaces

The adsorption properties of structurally well defined bimetallic Pt/Ru(0001) surfaces, consisting of a Ru(0001) substrate partly or fully covered by monolayer Pt islands or a monolayer Pt film, were studied by temperature programmed desorption (TPD) using CO and deuterium as probe molecules. Additionally, the adsorption of CO was investigated by infrared reflection absorption spectroscopy (IRAS). The presence of the pseudomorphic platinum islands or monolayer film leads to considerable modifications of the adsorption properties for both adsorbates, both on the Pt covered and, to a smaller extent, on the bare Ru part of the surfaces. In addition to distinct weakly bound adspecies, which are adsorbed on the monolayer Pt islands, we find unique contributions from island edge desorption, from spill-over processes during the desorption run, and a general down-shift of the peak related to desorption from Pt-free Ru(0001) areas with increasing Pt coverage. These effects, which we consider as characteristic for adsorption on bimetallic surfaces with large contiguous areas of the respective types, are discussed in detail.     

Superhydrophobic effects of self-assembled monolayers on micropatterned surfaces: 3-D arrays mimicking the lotus leaf

The contact angle of water has been measured on a series of self-assembled monolayers (SAM) on thermally evaporated and sputter coated silver surfaces. It is found that micropatterning the surface using nanosphere lithography leads to large increases in the contact angle and generates superhydrophobic surfaces with contact angles >150 degrees. The type of functional groups on the SAMs, the metal island size, and the metal island thickness all contribute to the measured contact angle. The maximum contact angle found was 161 degrees for a fluorinated alkanethiol on 80 nm thick silver islands.     

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.     

Pressure-dependent Ni-O phase transitions and Ni oxide formation on Pt(111): an in situ STM study at elevated temperatures

Growth, atomic structure and O2 partial pressure dependent phase transitions of Ni-O structures and thin NiO films on Pt(111) have been studied using scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). In situ STM experiments were performed during film growth by reactive metal deposition at elevated temperatures (400-550 K) and variable O2 pressure. Depending on the substrate temperature, one-dimensional network-like Ni-O structures and islands with (7x1) and (4x2) reconstructions are formed during the initial stages of growth. These structures transform reversibly to a (2x2) reconstruction in a narrow O2 pressure range of 1.5-2x10(-6) mbar and can be monitored by in situ STM. Upon reduction of the O2 pressure to <10(-10) mbar pseudomorphic Ni monolayers are obtained. The defect-free ordering of Ni atoms on Pt(111) in a single stacking domain indicates an O-surfactant induced growth mode. The structural properties of the O2 pressure-dependent Ni-O phases are discussed in a simple model assuming NiO(001)-like atomic arrangements in the adsorbate overlayer. At higher coverage stable (111)-oriented NiO islands grow in a three-dimensional mode.     

Evolution of surface width in electrochemical nucleation and growth

The evolution of surface width during growth of copper islands is studied in the kinetic regime, allowing control of island shape as well as island density. We show that film roughness at island coalescence is determined by island density, island shape, and the island growth kinetics. The connection between the kinetics of island growth and film growth provides the scientific basis for control of surface roughness.