Ag Island Films Research Articles

Mapping single‐molecule SERRS from Langmuir–Blodgett monolayers on nanostructured silver island films

AbstractTo explore the breakdown of ensemble averaging as the single molecule regime is approached, spatial mapping of surface‐enhanced resonance Raman scattering (SERRS) intensities was employed in the study of mixed dye–fatty acid Langmuir–Blodgett (LB) monolayers deposited on nanostructured Ag island films. By variation of the ratio of the two components in the films, the effects of dye concentration, on both SERRS spectra and LB monolayer architecture, were explored down to the single‐molecule level. Insight was gained into the nature of areas of intense local electromagnetic field strength (i.e. ‘hot spots’) that provide enormous enhancement of Raman signals, and their wavenumber of occurrence on nanostructured Ag island films. These enhancing films were further characterized by UV–visible surface plasmon absorbance and atomic force microscopy. The target analyte employed in this work, n‐pentyl‐5‐salicylimidoperylene, was dispersed in monolayers of arachidic acid on Ag nanostructured films for single point and 2D mapping SERRS experiments. The optical characterization of this dye was completed with solution‐phase molecular absorption and fluorescence, in addition to monolayer resonance Raman scattering (RRS). Single‐molecule SERRS mapping results suggest that electromagnetic hot spots on Ag island films are in fact highly localized on the nanoscale, corresponding to relatively few molecular sites, and hence also stress the rarity of coincidence between hot spots and single isolated molecules as a key consideration in ultrasensitive SERRS measurements of this type. Copyright © 2005 John Wiley & Sons, Ltd.

Raman Microspectroscopic Study on Polymerization and Degradation Processes of a Diacetylene Derivative at Surface Enhanced Raman Scattering Active Substrates. 1. Reaction Kinetics

Raman microspectroscopy was applied to study the polymerization kinetics of the Langmuir-Blodgett (LB) films of 10,12-pentacosadiynoic acid (DA) adsorbed on surface enhanced Raman scattering (SERS) active Ag island films. A two-dimensional (2D) Raman microscopic image measured at 1520 cm(-1) exhibits bright and dim spots with the diameter of several hundred nanometers. Raman microscopic spectra, measured by defocusing the excitation laser light (532 nm, diameter of ca. 10 mum) on the samples at room temperature, proved the occurrence of the surface processes consisting of the formation of polydiacetylene (PDA) in the blue phase, its conversion to the red phase, and subsequent bleaching. These reactions were negligible under the same condition for the DA-LB films prepared on a smooth (i.e., SERS-inactive) Ag film, indicating that the 532-nm-induced polymerization and the bleaching process are enhanced by the SERS-active substrates. At low temperatures below -50 degrees C, the Raman microscopic measurements proved the formation of the blue phase and its conversion to the red phase with much lower reaction rates compared to the corresponding rates at room temperature. The bleaching, however, was much suppressed at the low temperatures. The kinetic analyses of the formation of the blue phase and its conversion to the red phase were performed by using the intensity changes of the Raman bands due to the blue (1477 cm(-1)) and red (1517 cm(-1)) phases as a function of the irradiation time. The results strongly suggested the existence of at least two processes taking place simultaneously on the SERS-active substrates; that is, one of the processes is a sequential reaction, DA-monomers --> PDA in the blue phase --> PDA in the red phase, and the other is another sequential reaction, DA-monomers --> PDA in the red phase --> degradation species (probably amorphous carbon). Thus, even at the low temperatures, there occurs the surface reaction consisting of the formation of PDA and its degradation. The reaction can be ascribed to a process taking place at the highly SERS-active site, which gives the bright spot (so-called "hot spot") on the 2D Raman image, as proved by the confocal Raman microscopic measurement in the following paper.

Raman Microspectroscopic Study on Polymerization and Degradation Processes of a Diacetylene Derivative at Surface Enhanced Raman Scattering Active Substrates. 2. Confocal Raman Microscopic Observation of Polydiacetylene Adsorbed on Active Sites

Confocal Raman microscopic measurements were performed at room temperature on the Langmuir-Blodgett (LB) monolayer of 10,12-pentacosadiynoic acid (DA) prepared on surface enhanced Raman scattering (SERS) active Ag island films, two-dimensional (2D) Raman images of which exhibit bright and dim spots on a dark background. The measurements performed by focusing the excitation laser light (488 nm) on the dark background indicate the prompt appearance of the Raman bands (1515 and 2115 cm(-1)) due to polydiacetylene (PDA) in the red phase and subsequent diminution of the Raman bands. On the other hand, the spectra observed by focusing the excitation laser spot on the dim and bright spots exhibit almost random fluctuations, giving rather narrow Raman bands in the 1620-1000 cm(-1) region, which appear and disappear temporarily with varying intensities under the continuous irradiation at 488 nm. Broad Raman bands appear around 1580 and 1360 cm(-1), which are ascribable to amorphous carbon, at a later stage of the observation, the intensities from the bright spot being more than 100 times stronger than those from the dim spot. The narrow bands are ascribed to a series of carbonaceous intermediates such as polyenes, graphite sheets with various sizes, and folded or reorganized forms of the sheets including carbon nanotubes and fullerenes, which are formed during the conversion of PDA to amorphous carbon. The random spectral fluctuation was interpreted by considering that the intermediates undergo thermally activated diffusion and get temporarily in contact with the SERS-active site, resulting in the enhancement of their Raman bands and the fluctuation.

Surface enhanced Raman scattering to study surface contaminants on semiconductors

Abstract This work shows that by using surface enhanced Raman spectroscopy (SERS) it is possible to observe and characterize surface species due to contamination onto semiconductor materials. We detected the presence of surface contaminants on semiconductors by coating the surface with an Ag island film. By means of this technique we were able to observe the Raman bands of these contaminants, enhanced through the surface enhanced scattering mechanism. These results demonstrate the high sensitivity of SERS and suggest surface contaminants can be detected by SERS, even if they are present at very low concentration. This extension of SERS spectroscopy to semiconductor materials is important for developing SERS as a new surface sensitive probe in material science.

Origin of Enhanced Fluorescence in J-Aggregate-Coated Sputtered Ag Island Film

Origin of Enhanced Fluorescence in J-Aggregate-Coated Sputtered Ag Island Film

Development of A Novel Dual-Layer Thick Ag Substrate for Surface-Enhanced Raman Scattering (SERS) of Self-Assembled Monolayers

A dual-layer, thick (∼70 nm) vapor-deposited Ag substrate has been developed that gives enhancement factors on the order of 104 for surface-enhanced Raman scattering (SERS) experiments. This substrate has a total thickness of ∼70 nm but also has an outermost surface morphology that approximates that of a thin Ag island film (AgIF) substrate. This is accomplished using a dual overlayer/underlayer structure in which a thick underlayer of 45-nm Ag is vapor-deposited onto a treated glass slide. This Ag underlayer is exposed to ambient conditions under which the surface chemisorbs oxygen, leading to the thermodynamically favorable formation of an active Ag2O interface. An overlayer of 25-nm Ag is vapor-deposited on top of this structure. The first Ag/Ag2O underlayer produces an active interface that decreases the diffusion of the Ag atoms from the second vapor-deposited overlayer, thereby forming Ag particles with shapes favorable for SERS enhancement. Atomic force microscopy results show that the Ag overlayer particles have ideal shapes for SERS enhancement with morphology comparable to thin, vapor-deposited AgIFs. X-ray photoelectron spectroscopy in the O(1s) region showed the presence of multiple forms of oxygen in the Ag/Ag2O underlayer; the main forms were identified as dissolved bulk oxygen and chemisorbed oxygen. Using self-assembled monolayers (SAMs) of 1-dodecanethiol, a SERS intensity increase of ∼400% was obtained for this new overlayer/underlayer Ag substrate when compared to SAMs formed on traditional thick Ag substrates. Enhancement factors calculated from the Raman intensity of trans-1,2-bis(4-pyridyl)ethane showed a SERS enhancement of approximately 104 for this new Ag/Ag2O/Ag substrate compared with the bulk. This compares favorably with the SERS enhancements obtained using thin AgIF substrates. The Ag/Ag2O/Ag substrates showed reproducible SERS intensities (RSD = 0.45−5%). The mechanisms responsible for the overall enhancement on this new substrate are proposed to be the ideal surface morphology of the Ag overlayer particles as well as a combined enhanced electromagnetic field produced by both the Ag/Ag2O underlayer and the Ag overlayer.

Single molecular detection of a perylene dye dispersed in a Langmuir–Blodgett fatty acid monolayer using surface-enhanced resonance Raman scattering

The Langmuir–Blodgett (LB) monolayer technique was used to fabricate single molecule LB monolayer containing bis(phenethylimido)perylene (PhPTCD), a red dye dispersed in arachidic acid (AA) with an average doping of 1 molecule per μm 2. The monolayer was transferred onto Ag island films to obtain spatially resolved surface-enhanced resonance Raman scattering (SERRS) spectra. The mixed LB monolayers were fabricated with a concentration, on average, of 1, 6, 19 and 118 PhPTCD molecules per μm 2 in AA. The AA provides a two-dimensional host matrix whose background signal does not interfere with the detection of the probe molecule's SERRS signal. The properties of the single molecule detection were investigated using micro-Raman with a 514.5-nm laser line. The Ag island surfaces coated with the LB monolayer were mapped with spatial steps of 3 μm and global chemical imaging of the most intense SERRS band in the spectrum was also recorded. The SERRS and surface-enhanced fluorescence (SEF) of the neat and single molecule LB monolayer were recorded in a temperature range from liquid nitrogen to +200°C. Neat PhPTCD LB monolayer spectra served as reference for the identification of characteristic signatures of the single molecule behavior. The spatial resolution of Raman-microscopy experiments, the multiplicative effect of resonance Raman and SERRS, and the high sensitivity of the new dispersive Raman instruments, allow SERRS to be part of the family of single molecular spectroscopies.

Vanadyl naphthalocyanine and vanadyl porphine phenyl substituted macrocycles: SERS and thin film organisation studies

Infrared (middle and far) and Raman spectroscopy were used to study the adsorption of vanadylnaphthalocyanine and vanadylporphine tetraphenyl substituted macrocycles adsorbed onto metallic surfaces. The IR spectra of the compounds deposited onto a KBr monocrystal and onto a smooth copper surface suggest a weak adsorbate–substrate interaction and no significant structural modifications imposed by surface effect. The reflection–absorption IR (IRRAS) spectra allowed to propose a preferential molecular orientation of the molecules deposited onto the copper surface. Surface-enhanced Raman spectral data obtained on colloidal Ag as well as Ag island films suggest a weak macrocycle interaction and small structural modifications of the naphthalocyanine complex on the surface. The whole spectral data indicate that the naphthalocyanine complex is orientated with the naphthalocyanine plane face-on to the surface. In both complexes the vanadyl group is perpendicular to the coordination site and opposed to the surface. In both molecular systems the phenyl substituents, oriented perpendicular to the macrocycle plane, are responsible of the weak adsorbate–substrate interaction. Porphine complex has no interaction with the surface. This different behaviour of the above complexes is related to the different extent of the π-electronic size.

ATR-SNOM-Raman spectroscopy

In this study, (1) an ATR-SNOM-Raman equipment was built to obtain topography and SNOM images simultaneously, (2) Raman spectra from copper phthalocyanine (CuPc) were obtained with 600 s without using the resonance effect, (3) the optimum conditions to excite the LSP (Localized Surface Plasmon) on the Ag island films were found, (4) the Raman signal from CuPc (1.5 nm) was enhanced by a factor of ca. 300 using the Ag island film (5 nm), which gives 36 counts/s for ca. 7000 molecules assuming a sampled volume of φ 50 nm×1.5 nm thickness, and enables us to obtain the Raman spectra in 2 s. At last, (5) SNOM-Raman signal arises from the surface < ca. 2 nm similar to a localized SPP field.

Attenuated Total Reflection-Scanning Near-Field Raman Spectroscopy

A facility for scanning near-field Raman spectroscopy (SNRS) using the attenuated total reflection (ATR) method was built to obtain topographic and Raman images with a spatial resolution higher than the diffraction limit. A Raman spectrum of copper phthalocyanine (CuPc) was obtained with an accumulation time of 1000 s without using surface plasmon excitation or the electronic resonance effect of the molecules. The optimum conditions for enhancing the Raman signal using localized surface plasmon (LSP) on Ag island films were found to be 5 nm in thickness and an incident angle of 50°. Under these conditions, the Raman signal from CuPc (1.5 nm) was enhanced by a factor of ca. 300, which gives 36 counts/s for ca. 30,000 molecules, and enables us to obtain the Raman spectra in 2 s. Interestingly, the scanning near-field optical microscopy (SNOM)-Raman intensity increases with decreasing thickness of CuPc samples below 3 nm due to the near-field effect between the fiber probe and Ag films, while it remains constant for 5–30 nm in CuPc thickness. The polarization dependence of the signal intensity supports this interpretation. In contrast, without the Ag underlayer, the SNOM-Raman scattering intensity is not prominently enhanced due to the weaker interaction between the dielectric sample and the probe. In both cases, the SNOM-Raman scattering signal arises within ca. 5 nm from the sample surface.

Effects of silver particles on the photovoltaic properties of dye-sensitized TiO 2 thin films

To evaluate the possibility of using the plasmon resonance effect to enhance the efficiency of photochemical cells, cis-(SCN) 2Bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) dye-sensitized cells were used to measure the photoresponse of TiO 2 film electrodes before and after deposition of Ag particles. The deposited Ag particles created a film with Ag islands. We found that the photoresponse in the visible region increased as the mass-equivalent Ag-island film thickness, t Ag, increased to 3.3 nm, but decreased when t Ag was further increased to 6 nm. On the other hand, compared with bare TiO 2 films, the photoresponse in the UV region decreased for any level of Ag islands. These results suggest that under proper conditions, enhancement of the optical absorption of the dye by the Ag plasmon resonance effect contributes to the photocurrent, and indicates the possibility of improving the energy conversion efficiency of photoelectrochemical cells with Ag-island films.

Optical absorption of continuous-network-structure Ag and Ir films: Presence of particlelike and conduction-electron-localized regions

Optical absorption of continuous-network-structure (CNS) Ag and Ir films has been measured in the photon-energy range of 0.5--6.5 eV. As absorption due to conduction electrons, optical plasma-resonance absorption (OPRA), reported for Ag island films consisting of Ag particles, and weak Drude-type absorption (DTA) were found for the CNS Ag films, and only the weak DTA appeared in the CNS Ir films. The finding of the OPRA indicates that particlelike regions, where plasma oscillations of conduction electrons occur as in Ag particles, are present in the CNS Ag films. When the particlelike regions are formed, the region, where DTA occurs, is reduced. This reduction results in the weakening of the DTA. Thus the weak DTA supports the presence of the particlelike regions. If particlelike regions are present in the CNS Ir films also, conduction-electron localization as in Ir particles should occur in the particlelike regions. In this case, only the DTA, weak as in the CNS Ag films, must appear. This agrees with the above appearance. Thus conduction-electron-localized regions are present in the CNS Ir films.

IR absorption enhancement for physisorbed methanol on Ag island films deposited on the oxidized and H-terminated Si(111) surfaces: effect of the metal surface morphology

Infrared absorption measurements using a multiple internal reflection geometry are reported for condensed methanol at 90 K on Ag island films deposited on the oxidized and hydrogen-terminated surfaces of Si(111). The attenuated total reflection (ATR) spectra obtained as a function of methanol exposure (up to 14 L) show that a 1-nm mass thickness of Ag island film on the oxidized Si(111) surface yields an absorption intensity 2–3 times larger than the intensity in the absence of Ag on the oxidized surface. Deposition of the same thickness of Ag on the hydrogen-terminated Si(111) surface results in approximately twice the enhancement. The different magnitudes of the enhancement are discussed based on SEM micrographs for Ag island films formed on the oxidized and H-terminated Si(111) surfaces.

STM study of nucleation of Ag on Si(111)−(7 × 7) surface

Initial stages of Ag on Si(111)−(7 × 7) surface nucleation were studied at submonolayer coverage. Samples were prepared by thermal evaporation of Ag from tungsten wire under UHV conditions (p<2.5 × 10−8 Pa). Various deposition rates (0.002–0.1 ML s−1) were used to prepare Ag island films with coverages (0.002–2) ML (1 ML ≈ 7.58 × 1014 atoms cm−2) at room temperature. We observed preferential growth on faulted half unit cells (F cells). At constant coverage both the island density and ratio of occupied F and U (unfaulted) cells are independent of the deposition rate, which is an evidence for dominant influence of substrate structure. The preference of nucleation in the F cells against U cells decreases with the coverage until the ratio is 1:1 for 1 ML Ag film. We have observed that presence of an Ag island in any type of the half unit cell (F or U) considerably reduces nucleation probability in neighbouring cells. This results in forming of structural patterns observed among randomly grown Ag-islands which is a new feature found for Ag/Si(111)−(7 × 7) system.

SERS spectroscopy to study adsorbates on semiconductor surfaces

SERS spectra were observed from molecules adsorbed on silver and on various semiconductor surfaces on to which an Ag island film had been evaporated. Because of the observed spectral differences between these spectra, it could be confirmed that the observed Raman scattering arises from molecules adsorbed on semiconductors and not on the Ag film. With this method, Raman scattering from the surface of the semiconductors is enhanced because the discontinuous Ag film produces a large electromagnetic enhancement at each Ag particle. Therefore, the surface sensitivity of the SERS effect at metal surface was used to study molecules adsorbed on the semiconductors. It was not possible to detect Raman scattering from surfaces without the deposited Ag film because the vibrational spectra of molecules adsorbed on the semiconductor surfaces were too weak to be detected. These results prove that adsorbed species on semiconductor surface can be detected by SERS spectroscopy by coating the surface with a discontinuous Ag film. © 1998 John Wiley & Sons, Ltd.

DISPERSION BEHAVIOR OF HEMATOPORPHYRIN IN MONOMOLECULAR FILM OF STEARIC ACID

ABSTRACT Dispersion characteristics of the monomolecular films of hematoporphyrin mixed with stearic acid has been studied. The average molecular areas of mixed monolayers coincided approximately with the ideal additivity rule when the“solid” films were extrapolated to zero surface pressure. The orientation of the macrocycles was given by infrared dichroism with the tilting angle of 10. 1° to the plane of the film. The SERS of LB films deposited on silver island films was observed- The results showed that there are two possible contribution, i. e., electromagnetic and chemical interaction between the hematoporphyrin molecules and Ag island films.

Optical properties of Ag island films prepared by radio-frequency magnetron-sputtering using attenuated total reflection method

Abstract The optical properties of Ag island films deposited on glass substrates at room temperature by rf magnetron-sputtering have been investigated. The reflection of p- and s-polarized laser light with a wavelength of 633 nm by the Ag thin films was measured in an attenuated total reflection (ATR) configuration. The effective dielectric constants ∊ and effective optical thickness d opt of the films were obtained from theoretical fitting of the ppolarized ATR data. The measured ATR curves were very sensitive to metal island structure. For films with a thickness of less than about 16 nm, ∊ was very dependent upon d opt and both the real and imaginal parts of ∊ increased considerably for thinner films.

Optical properties of Ag island films prepared by radio-frequency magnetron-sputtering using attenuated total reflection method

The optical properties of Ag island films deposited on glass substrates at room temperature by rf magnetron-sputtering have been investigated. The reflection of p- and s-polarized laser light with a wavelength of 633 nm by the Ag thin films was measured in an attenuated total reflection (ATR) configuration. The effective dielectric constants e and effective optical thickness d opt. of the films were obtained from theoretical fitting of the p-polarized ATR data. The measured ATR curves were very sensitive to metal island structure. For films with a thickness of less than about 16 nm, e was very dependent upon d opt. and both the real and imaginal parts of e increased considerably for thinner films.

Raman spectroscopy of adsorbates on semiconductors by means of SERS effect

An application of surface enhanced Raman scattering (SERS) to the surface characterization of semiconductors is reported by evaporating an Ag island film onto the surface of the materials. The surface sensitivity of the Raman effect at the metal surface is used for the study of molecules absorbed onto the semiconductors. With this method there is enhanced Raman scattering from the surface because the discontinuous Ag layer produces large electromagnetic enhancement at each Ag particle. SERS spectra are observed from tris(bipyridine)ruthenium, Ru(bpy) 3 2+ , molecules adsorbed on Si, GaAs, InP, silver, and glass substrates onto which an Ag island film has been evaporated. The spectral differences observed between the spectra from the molecules adsorbed on the semiconductors and on the glass and silver substrates allow us to affirm that the SERS signals arise from the substrate surface and not from the Ag island film. Raman scattering cannot be detected from surfaces without the deposited Ag film because the vibrational spectra of molecules adsorbed on the surface are too weak to be detected. These results prove that by using the Ag overlayer method is should be possible to study the properties of adsorbed species on semiconductor surfaces. This suggests the possibility of utilizing the SERS effect as a new surface probe of the semiconductor surface considering that Raman spectroscopy can give detailed information about the state of chemical species on the surface.

Characterization of Phthalocyanine Langmuir-Blodgett Films by Optical Dichroism and Resonance Raman Scattering

Abstract In this Paper we report optical dichroism and polarized resonance Raman scattering measurements on Langmuir-Blodgett layers of tetra tertiary butyl monosulphonic acid of phthalocyanine (tBPcSA). These measurements were performed to determine molecular alignment and local ordering in our samples, using surface enhanced optical and Raman spectroscopy. Thus our L-B layers were deposited on rough thin (50 Å) Ag island films. We found that the surface enhancement effects dominated the optical behaviour of our samples, making it more difficult to extract the desired information on molecular orientation. At the same time, we found that tBPcSA L-B layers can be a good system to study resonance effects and to test the reliability of the structural information obtainable by optical spectroscopic techniques.