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Surface plasmon contribution to SERS
Abstract
The contribution of surface plasmon excitation to SERS has been experimentally investigated with molecules adsorbed on 5 and 57 nm evaporated Ag films coated on a hemicylindrical prism which enabled direct excitation of surface plasmons in the Kretschmann configuration. Surface plasmon excitation increases SERS intensities by at least 10 × while metal islands of Ag give much larger SERS intensities.
... SERS is of high demand by biochemists because of its in vivo sensing ability [3]. SERS exploits Raman scattering, known for exceptionally high molecular specificity, in combination with enhancement given by locally generated surface plasmons-collective oscillations of electrons-induced by the incident laser field [4]. The enhancement is critical because without it, spontaneous Raman scattering is extremely inefficient and requires a large number of molecules to produce a measurable scattering signal. ...
... The enhancement is critical because without it, spontaneous Raman scattering is extremely inefficient and requires a large number of molecules to produce a measurable scattering signal. Typically, two SERS mechanisms are identified as contributing to the Raman signal enhancement: chemical (charge transfer) mechanism, providing a ∼10 2 [5], and electromagnetic mechanism responsible for ∼10 4 −10 12 enhancement [6]. The total SERS enhancement factor has been observed to be as large as 10 14 [7]; however, quantitative theories on the sensitivity limit of SERS are still under debate [8], and the question of whether SERS is capable of sensing a single molecule is still open. ...
Surface-enhanced Raman scattering (SERS) spectroscopy is a popular technique for detecting chemicals in small quantities. Rough metallic surfaces with nanofeatures are some of the most widespread and commercially successful substrates for efficient SERS measurements. A rough metallic surface creates a high-density random distribution of so-called “hot spots” with local optical field enhancement causing Raman signal to increase. In this Letter, we revisit the classic SERS experiment [Surf. Sci. 158, 229 (1985)SUSCAS0039-602810.1016/0039-6028(85)90297-3] with rough metallic surfaces covered by a thin layer of copper phthalocyanine molecules. As a modification to the classic configuration, we apply an adaptive wavefront correction of a laser beam profile. As a result, we demonstrate an increase in brightness of local SERS hot spots and redistribution of Raman signal over the substrate area. We hypothesize that the improvement is due to optimal coupling of the shaped laser beam to the random plasmonic nanoantenna configurations. We show that the proposed adaptive-SERS modification is independent of the exact structure of the surface roughness and topography, works with many rough surfaces, and gives brighter Raman hot spots in comparison with conventional SERS measurements. We prove that the adaptive SERS is a powerful instrument for improving SERS sensitivity.
... This variation of Raman peaks resembles the SERS blinking effect [28], however, within a given void the spectrum does not change with time. In spite of the latter we assume that the observed phenomenon is arising from the same effects, as blinking, namely excitation of molecules into a metastable and non-emitting state [29,30]; photoionization affecting charge transfer states [31]; change in orientation of the molecules [32]; photoinduced electron transfer between the adsorbed molecule and the SERS surface [33]; diffusion of molecules at hotspots [34]; ...
... It is known that in addition to SERS, the surface enhancement works for PL process as well [9]. However, while the strongest contribution to SERS comes right from the plasmonic surface [29], the PL is quenched there, and the surface enhancement of photoluminescence has maximum at some distance. Therefore, from the spectra shown on Fig. 4 it can be concluded, that the strongest amplification in samples with larger GNPs is related to PL enhancement and comes from volumes being at some distance from the metallic surface. ...
Giant plasmonic surface enhancement has been observed in gold coated micron sized inverse pyramids entrapping a gold nanoparticle. The amplification of both surface enhanced Raman and photoluminescence signals was found to be dependent on the diameter of trapped gold nanoparticle and around 50-fold enhancement was detected for 250nm diameter sample relatively to the 50nm one. Finite differential time domain simulations, performed to determine the near-field distribution in the structure, showed that when the nanoparticle protrudes into the hotspot zone of the void, coupling of electromagnetic field occurs and the plasmon-related near-field enhancement is concentrated into the close vicinity of the nanoparticle, mainly into the close gaps around the tangential points of the curved sphere and the flat pyramid surface. This results in a more than 15 times increase of the near-field intensity, compared to the empty void.
... SERS is of high demand by biochemists because of its in vivo sensing ability [3]. SERS exploits Raman scattering, known for exceptionally high molecular specificity, in combination with enhancement given by locally generated surface plasmons-collective oscillations of electrons-induced by the incident laser field [4]. The enhancement is critical because without it, spontaneous Raman scattering is extremely inefficient and requires a large number of molecules to produce a measurable scattering signal. ...
... The enhancement is critical because without it, spontaneous Raman scattering is extremely inefficient and requires a large number of molecules to produce a measurable scattering signal. Typically, two SERS mechanisms are identified as contributing to the Raman signal enhancement: chemical (charge transfer) mechanism, providing a ∼10 2 [5], and electromagnetic mechanism responsible for ∼10 4 −10 12 enhancement [6]. The total SERS enhancement factor has been observed to be as large as 10 14 [7]; however, quantitative theories on the sensitivity limit of SERS are still under debate [8], and the question of whether SERS is capable of sensing a single molecule is still open. ...
We utilize laser-driven molecular oscillations to modulate light and produce a broadband spectrum of mutually coherent Ramansidebands, aiming toward synthesis of ultrafast optical waveforms with prescribed temporal and spatial shapes.
... These TM nanostructures yield average enhancements ranging from one to three orders of magnitude. (29,48) It was Moskovits, (79) Creighton, (51) Chen, (52) Chang, (80,81) Pettinger, (50) Gersten, (82,83) Gersten and Nitzan, (84,85) McCall, (86,87) Kerker, (88 -90) Metiu, (22,91,92) and Schatz, (68,93,94) who majorly contributed to the development of SPR mechanism working for SERS from the late 1970s to mid-1980s. (19) In 1977, Moskovits and Hulse (95) studied the mechanism of the colors of the electrodeposited Al 2 O 3 and attributed it to the embedded transition metallic colloids with effective medium theory. ...
... Experimentally, Chang et al. compared ATR-Raman from Ag-film-coated hemicylindrical prism with surface Raman spectra from Ag islands, and they demonstrated the contribution of surface plasmon excitation to SERS in 1979. (80) Pettinger et al. (50) observed the strong dependence of surface Raman intensity on exciting frequency and the angle of incident light for roughened Au, Ag, and Cu electrodes and obtained the evidence for the surface plasmon origin of SERS; Tsang et al. (103) reported similar results but SERS on the surfaces of Ag grating. ...
Surface-enhanced Raman scattering (SERS) was discovered in the mid-1970s, by which the intrinsically low detection sensitivity of Raman spectroscopy is no longer a fatal disadvantage for this analytical tool. As a general introduction of SERS, the almost 40-year history of SERS is first overviewed, showing that SERS has gone through a tortuous pathway to develop into a powerful diagnostic technique. We then describe the principle of SERS and enhancement mechanisms, illustrating that SERS is mainly surface plasmon resonance (SPR)- and nanostructure-enhancement phenomenon. The SERS measurement procedures, in particular the preparation of various SERS active substrates, are discussed. On the basis of four important criteria in analytical science, i.e. detection sensitivity, (energetic, spatial, and temporal) resolution, generality, and reliability, we highlight two different approaches to utilize the strength and offset the weakness of SERS. With the enormously high sensitivity and spectral resolution, SERS has been applied successfully to surface analysis and trace analysis by gaining meaningful information from an extremely small quantity of species even down to single molecules. To significantly improve the surface generality and spatial resolution, tip-enhanced Raman spectroscopy (TERS) was invented in 2000. To greatly improve the material generality and measurement reliability, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was introduced in 2010. Finally, prospective developments of SERS in substrates, methods, and theory are briefly discussed.Keywords: SERS ; TERS ; SHINERS ;surface plasmon; Raman Spectroscopy
... Surface plasmons (SP) are electron oscillations that exist on a metal-dielectric interface at the surface of the metal, whereas bulk plasmons (BP) can form deeper within the body of a free carrier containing system. SP can significantly enhance the performance of diverse applications ranging from optical, biological, and chemical sensors to surface-enhanced Raman spectroscopy and photovoltaic devices [133][134][135][136][137]. MXene exhibits high metallic conductivity, controllable hydrophilicity, chemical and environmental stability, and has scalable synthesis, which makes them ideal candidates for plasmonic device applications. ...
Two-dimensional (2D) layers of transition metal carbides, nitrides, or carbonitrides, collectively referred to as MXenes, are considered as the new family of 2D materials for the development of functional building blocks for optoelectronic and photonic device applications. Their advantages are based on their unique and tunable electronic and optical properties, which depend on the modulation of transition metal elements or surface functional groups. In this paper, we have presented a comprehensive review of MXenes to suggest an insightful perspective on future nanophotonic and optoelectronic device applications based on advanced synthesis processes and theoretically predicted or experimentally verified material properties. Recently developed optoelectronic and photonic devices, such as photodetectors, solar cells, fiber lasers, and light-emitting diodes are summarized in this review. Wide-spectrum photodetection with high photoresponsivity, high-yield solar cells, and effective saturable absorption were achieved by exploiting different MXenes. Further, the great potential of MXenes as an electrode material is predicted with a controllable work function in a wide range (1.6–8 eV) and high conductivity (~10 ⁴ S/cm), and their potential as active channel material by generating a tunable energy bandgap is likewise shown. MXene can provide new functional building blocks for future generation nanophotonic device applications.
... 1) M. Moskovits group suggested that this phenomenon could be caused by surface plasmon resonance (SPR), which is a collective oscillation of free electrons at the surface of metal nanostructures by an external light source. [2][3][4][5][6][7][8][9][10][11][12][13][14] After about 40 years, the SERS study has attracted great attention as a biomolecule analysis technology, and more than 2500 new papers and 500 review papers related to SERS topic have been published each year in recently. 15) The advantages of biomaterials analysis using SERS are as follows; ① Molecular level analysis is possible based on unique fingerprint information of biomolecule, [16][17][18][19][20] ② There is no photo-bleaching effect of the Raman reporters, allowing long-term monitoring of biomaterials compared to fluorescence microscopy, ③ SERS peak bandwidth is approximately 10 to 100 times narrower than fluorescence emission from organic phosphor or quantum dot, resulting in higher analysis accuracy, 21), 22) ④ Single excitation wavelength allows analysis of various biomaterials, ⑤ By utilizing near-infrared (NIR) SERSactivated nanostructures and NIR excitation lasers, auto-fluorescence noise in the visible wavelength range can be avoided from in vivo experiment and light damage in living cells can be minimized compared to visible lasers, ⑥ ...
... The aim of SERS is to enhance the Raman signal by placing molecules in the vicinity of metal nanoparticles or on a metal surface, which induce plasmonic resonance and strong signal enhancement when interacting with the incident light (Dornhaus et al. 1980). This technique was applied to study the influence of Cu(II) on the Aβ 1-16 fragment using a combination of SERS, electronic circular dichroism and scanning tunneling microscopy (STM). ...
It is well established that amyloid proteins play a primary role in neurodegenerative diseases. Alzheimer’s, Parkinson’s, type II diabetes, and Creutzfeldt-Jakob’s diseases are part of a wider family encompassing more than 50 human pathologies related to aggregation of proteins. Although this field of research is thoroughly investigated, several aspects of fibrillization remain misunderstood, which in turn slows down, or even impedes, advances in treating and curing amyloidoses. To solve this problem, several research groups have chosen to focus on short fragments of amyloid proteins, sequences that have been found to be of great importance for the amyloid formation process. Studying short peptides allows bypassing the complexity of working with full-length proteins and may provide important information relative to critical segments of amyloid proteins. To this end, efficient biophysical tools are required. In this review, we focus on two essential types of spectroscopic techniques, i.e., vibrational spectroscopy and its derivatives (conventional Raman scattering, deep-UV resonance Raman (DUVRR), Raman optical activity (ROA), surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), infrared (IR) absorption spectroscopy, vibrational circular dichroism (VCD)) and solid-state nuclear magnetic resonance (ssNMR). These techniques revealed powerful to provide a better atomic and molecular comprehension of the amyloidogenic process and fibril structure. This review aims at underlining the information that these techniques can provide and at highlighting their strengths and weaknesses when studying amyloid fragments. Meaningful examples from the literature are provided for each technique, and their complementarity is stressed for the kinetic and structural characterization of amyloid fibril formation.
... 3 Although RS has been discovered since the 1930s, it was not widely applied until the breakthrough discovery of surface-enhanced Raman scattering (SERS) in the 1970s, which led to a significant improvement of the signals with an enhancement factor (EF) of up to 10 5-6 when studying pyridine on rough silver electrodes. [4][5][6] The phenomenon of the SERS effect is now widely accepted by the contributions of both physical and chemical mechanisms which are electromagnetic field enhancement (EM) and chemical enhancement (CE), respectively. 7-11 EM field enhancement is due to the formation of localized surface plasma resonance (LSPR) on the nanostructures of metals, such as gold or silver. ...
In recent years, Surface Enhanced Raman Scattering (SERS) has been widely applied to many different areas, including chemical analysis, biomolecule detection, bioagent diagnostics, DNA sequence, and environmental monitor, due to its capabilities of unlabeled fingerprint identification, high sensitivity, and rapid detection. In biomicrofluidic systems, it is also very powerful to integrate SERS based devices with specified micro-fluid flow fields to further focusing/enhancing/multiplexing SERS signals through molecule registration, concentration/accumulation, and allocation. In this review, after a brief introduction of the mechanism of SERS detection on proteins, we will first focus on the effectiveness of different nanostructures for SERS enhancement and light-to-heat conversion in trace protein analysis. Various protein molecule accumulation schemes by either (bio-)chemical or physical ways, such as immuno, electrochemical, Tip-enhanced Raman spectroscopy, and magnetic, will then be reviewed for further SERS signal amplification. The analytical and repeatability/stability issues of SERS detection on proteins will also be brought up for possible solutions. Then, the comparison about various ways employing microfluidic systems to register, concentrate, and enhance the signals of SERS and reduce the background noise by active or passive means to manipulate SERS nanostructures and protein molecules will be elaborated. Finally, we will carry on the discussion on the challenges and opportunities by introducing SERS into biomicrofluidic systems and their potential solutions.
... In 1979, Chang and coworkers experimentally compared ATR-Raman from a Ag-film-coated hemi-cylindrical prism with SERS from Ag islands, demonstrating the contribution of surface-plasmon excitation to SERS. 72 Pettinger and coworkers observed the strong dependence of surface Raman intensity on the exciting frequency and on the angle of incidence of the exciting light for roughened Au, Ag and Cu electrodes, providing additional evidence for the surface-plasmon origin of SERS. 74 Finally, Tsang and coworkers reported similar results for SERS on Ag gratings 75 in which a SP polariton could be excited by direct illumination. ...
Surface-enhanced Raman spectroscopy (SERS) and related spectroscopies are powered primarily by the concentration of the electromagnetic (EM) fields associated with light in or near appropriately nanostructured electrically-conducting materials, most prominently, but not exclusively high-conductivity metals such as silver and gold. This field concentration takes place on account of the excitation of surface-plasmon (SP) resonances in the nanostructured conductor. Optimizing nanostructures for SERS, therefore, implies optimizing the ability of plasmonic nanostructures to concentrate EM optical fields at locations where molecules of interest reside, and to enhance the radiation efficiency of the oscillating dipoles associated with these molecules and nanostructures. This review summarizes the development of theories over the past four decades pertinent to SERS, especially those contributing to our current understanding of SP-related SERS. Special emphasis is given to the salient strategies and theoretical approaches for optimizing nanostructures with hotspots as efficient EM near-field concentrating and far-field radiating substrates for SERS. A simple model is described in terms of which the upper limit of the SERS enhancement can be estimated. Several experimental strategies that may allow one to approach, or possibly exceed this limit, such as cascading the enhancement of the local and radiated EM field by the multiscale EM coupling of hierarchical structures, and generating hotspots by hybridizing an antenna mode with a plasmonic waveguide cavity mode, which would result in an increased local field enhancement, are discussed. Aiming to significantly broaden the application of SERS to other fields, and especially to material science, we consider hybrid structures of plasmonic nanostructures and other material phases and strategies for producing strong local EM fields at desired locations in such hybrid structures. In this vein, we consider some of the numerical strategies for simulating the optical properties and consequential SERS performance of particle-on-substrate systems that might guide the design of SERS-active systems. Finally, some current theoretical attempts are briefly discussed for unifying EM and non-EM contribution to SERS.
... The plasmon is an oscillation of charge in the device, which itself creates a local oscillating electric field or light. This scattering of light has numerous applications, including improved photovoltaics [4][5][6] , surface-enhanced Raman spectroscopy [7][8][9] , and other uses such as SPASERs 10-12 . This paper attempts to draw a relationship between the geometry of the nanodevice and the resonance peak. ...
Plasmonic nanodevices are metallic structures that exhibit plasmonic effects when exposed to light, causing scattering and enhancement of that light. These plasmons makes it possible for light to be focused below the diffraction limit. Dark-field spectroscopy has been used to capture the scattering spectra of these structures in order to examine the scattering and resonant frequencies of the plasmons provided by the devices. The geometries of the devices change which wavelengths of light are most readily able to couple to the device, resulting in a change in the wavelength of the scattered light. A variety of device geometries and configurations will be studied, including nanodiscs, nanowires, and plasmonic gratings, along with double-width nanogap plasmonic gratings. These new structures will have features below the fabrication limit of electron-beam lithography, i.e. sub-10 nanometer features. The polarization dependencies of these resonance modes are investigated as well. A relation between device geometry and wavelength will be drawn; in effect, this will allow the selection of geometry of the fabricated device based on the desired wavelength of light to be scattered. Preliminary Raman spectroscopy will also be performed in order to study the device response and usefulness for surface-enhanced Raman spectroscopy.
... Using Eq.(18), the Raman scattering intensity of the adsorbed molecule is compared with that of a free molecule. Since the Raman scattering of a free molecule is simply obtained from Eq.(18) by neglecting L m , the degree of the enhancement is given by fm(Wl-w2+ se-Sg) ] fph(wl-w2) (24) At the induced resonance situation where wl+ Sg < Sf and w2=wl-wph' we obtained ...
Mounting experimental observation of SERS has led to considerable theoretical efforts towards the understanding of various aspects of this phenomenon. The existing theories should be tested by their ability to explain the following important properties of SERS: (1) The Raman intensity of adsorbed molecules is enhanced by a factor of 105–106 compared to that of free molecules, (2) The degree of the enhancement depends on the excitation energy and also on the properties of the substrate metal, (3) The enhanced Raman scattering spectra are accompanied by a broad continuum spectrum, (4) The roughness of a metal surface plays a crucial role.
... Surface enhanced florescence and surface enhanced Raman scattering (SERS) [126,127,128] can provide an indirect mapping of SPPs. Plasmon propagation along nanowires can be visualized by covering the structure by a layer of fluorescence molecules [16,123,129,130,131] (Fig. 1.9(b)) or by decorating the nanowire surface with quantum dots [103,112,132,133]. ...
Plasmonic circuitry is considered as a promising solution-effectivetechnology for miniaturizing and integrating the next generation ofoptical nano-devices. The realization of a practical plasmonic circuitry strongly depends on the complete understanding of the propagation properties of two key elements: surface plasmons and electrons. The critical part constituting the plasmonic circuitry is a waveguide which can sustain the two information-carriers simultaneously. Therefore, we present in this thesis the investigations on the propagation of surface plasmons and the co-propagation of surface plasmons and electrons in single crystalline metal nanowires. This thesis is therefore divided into two parts. In the first part, we investigate surface plasmons propagating in individual thick penta-twinned crystalline silver nanowires using dual-plane leakage radiation microscopy. The effective index and the losses of the mode are determined by measuring the wave vector content of the light emitted in the substrate. Surface plasmon mode is determined by numerical simulations and an analogy is drawn with molecular orbitals compound with similar symmetry. Leaky and bound modes selected by polarization inhomogeneity are demonstrated. We further investigate the effect of wire geometry (length, diameter) on the effective index and propagation losses. On the basis of the results obtained during the first part, we further investigate the effect of an electron flow on surface plasmon properties. We investigate to what extend surface plasmons and current-carrying electrons interfere in such a shared circuitry. By synchronously recording surface plasmons and electrical output characteristics of single crystalline silver and gold nanowires, we determine the limiting factors hindering the co-propagation of electrical current and surface plasmons in these nanoscale circuits. Analysis of wave vector distributions in Fourier images indicates that the effect of current flow on surface plasmons propagation is reflected by the morphological change during the electromigration process. We further investigate the possible crosstalk between co-propagating electrons and surface plasmons by applying alternating current bias
... These plasmon modes have the high sensitivity of plasmon resonances to the environment. In the last decade, noble metal nanoparticles have attracted considerable attention because they have found wide applications such as surface enhanced Raman spectroscopy [11], biosensors [12] and nanolithography [13]. If the SIL is combined with the Au nanoparticles, we could achieve high-quality surface plasmon imaging. ...
A super-hemispherical (i.e. a truncated spherical) glass lens with gold (Au) nanoparticles was obtained using a surface tension mold (StM) technique. Recently, surface plasmon of noble metal nanoparticle has attracted a considerable amount of interest because it is extremely sensitive to the properties of the materials attached to its surface. On the other hand, in the field of high-resolution microscopy, solid immersion lenses (SILs) with super-hemispherical shape have received much attention because it is a convenient and powerful means of improving both the spatial resolution and the light collection efficiency. A combination of the SIL and the Au nanoparticles could be very suitable for use in surface plasmon microscopy. In this study, Na2O-CaO-SiO2 glass was heated on Au-coated glassy-carbon substrate up to 800 °C. The obtained glasses were found to have super-hemispherical shape, and the Au nanoparticles were deposited on its bottom planar surface. The effects of the deposition condition of Au on the distribution of Au nanoparticles and the shape of glass were investigated, and the surface plasmon resonance absorption spectra from the obtained samples were measured.
The surface‐enhanced Raman scattering effect refers to the effect was discovered in the mid‐1970s, by which the intrinsically low detection sensitivity of Raman spectroscopy is no longer a fatal disadvantage for this analytical tool. As a general introduction of surface‐enhanced Raman spectroscopy (SERS), the over 40‐year history of SERS is first overviewed, showing that SERS has gone through a tortuous pathway to develop into a powerful diagnostic technique. We then describe the principle of SERS and enhancement mechanisms, illustrating that SERS is mainly surface plasmon resonance (SPR) and nanostructure‐enhancement phenomenon. The SERS measurement procedures, in particular the preparation of various SERS active substrates, are discussed. Based on the four important criteria in analytical science, i.e. detection sensitivity (energetic, spatial, and temporal), resolution, generality, and reliability, we highlight two different approaches to utilize the strength and offset the weakness of SERS. With the enormously high sensitivity and spectral resolution, SERS has been applied successfully to surface analysis and trace analysis by gaining meaningful information from an extremely small quantity of species even down to single molecules. To significantly improve the surface generality and spatial resolution, tip‐enhanced Raman spectroscopy (TERS) was invented in 2000. To greatly improve the material generality and measurement reliability, shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS) was introduced in 2010. Finally, prospective developments of SERS in substrates, methods, and theory are briefly discussed.
Multibranched silver nanostructures (AgNSts) have attracted much attention as promising candidates for surface‐enhanced Raman spectroscopy (SERS) due to their unique plasmonic properties. We have chemically synthesized silver nanodendrites (AgNDs) and silver stars (AgSs) and investigated their SERS performance for trace‐level molecule detection. High enhancement factor (EF) ~10 ¹⁰ and attomolar detection limits were obtained with multibranched nanostructure‐based SERS substrates for methylene blue, thiram, and phosmet. The confinement of the local fields at the sharp tips and in the intrabranch and interbranch nanogaps of AgNDs offer highly dense three‐dimensional (3D) SERS “hot spots” and large signal enhancements of ~10 ⁹ . Further, the as‐prepared AgND‐based substrates were utilized for the ultrasensitive identification of explosive molecules 2,4‐DNT, PNBA, and PA with limit‐of‐detection (LOD) down to ~5.3 × 10 ⁻¹⁶ , 2.9 × 10 ⁻¹⁶ , and 3.8 × 10 ⁻¹² M concentrations, respectively. The spectral modifications and appearance of new SERS peaks in the low wavenumber region indicate the metal–molecule complex formation and confirm the contribution of the chemical enhancement mechanism. The narrow spectral widths of Raman peaks allow the highly selective multiplexed detection of explosive molecules from the two‐component (2‐plex) mixture of dyes with different concentrations. Further, the density functional theory (DFT)‐based Raman spectrum calculations of the molecules exhibit a reasonably good correspondence with the experimental results. Therefore, the molecularly specific and distinguishably sharp Raman features enable the ultrasensitive and multiplexed detection of analytes molecules with our AgNSt‐based SERS substrates.
Spherical high-index nanoparticles with low material losses support sharp high-Q electric and magnetic resonances and exhibit a number of interesting optical phenomena. Developments in fabrication techniques have enabled further study of their properties and the investigation of related optical effects. After deposition on a substrate, the optical properties of a particle change dramatically due to mutual interaction. Here, we consider a silicon spherical nanoparticle on a dielectric one-layered substrate. At normal incidence of light, the layer thickness controls the contribution of the nanoparticle's electric and magnetic multipoles to the subsequent optical response. We show that changing the polarization of incident light at a specific excitation angle and layer thickness leads to switching between the multipoles. We further observe a related polarization-driven control over the direction of the scattered radiation.
We sketch the life and scientific legacy of Prof. Richard K. (Kounai) Chang (张国鼐), 1940-2020, the Henry Ford II Professor of Applied Physics, Professor of Physics and Electrical Engineering at Yale University. We briefly outline his research from linear and non-linear optics; light interactions with solids and liquids in various surface, micro-shapes and nano-structures; optical effects and processes in microresonators; and later to laser spectroscopic characterization of sprays, combustion, and environmental pollutants. Prof. Chang has many colleagues, collaborators, students, and friends among the JQSRT community.
Significant increase of plasmonic surface enhancement has been observed in gold coated micron sized inverse pyramids entrapping a gold nanoparticle. The amplification of both surface enhanced Raman and fluorescence signals was found and it was dependent on the size of the trapped gold nanoparticle. The enhancement was found to be 50 times higher for the 250nm diameter nanosphere, compared to the 50nm one. Finite differential time domain simulations were performed to determine the near-field distribution in the structure and showed that when the nanoparticle protrudes into the hotspot zone of the void, coupling of the electromagnetic field occurs and the plasmon-related near-field enhancement is concentrated into the close vicinity of the nanoparticle, mainly into the gaps around the contact points of the curved sphere and the flat pyramid surfaces.
Propagating surface plasmons in the laboratory can have basic properties quite different from those of the ideal surface plasmon polariton, which is a bound mode at the interface between two infinite media. Features such as field confinement and propagation length are affected by the configuration used for coupling to the far field. We experimentally investigate how the Otto and Kretschmann configurations influence the linear coupling properties of smooth metal layers and the nonlinear generation of the second harmonic. The results are compared to calculations from electrodynamic theory based on the hydrodynamic model, which confirm that the nonlinear yield is an order of magnitude greater in the Kretschmann configuration than in the Otto configuration.
Core-shell nanoparticles are at the leading edge of the hot research topics and offer a
wide range of applications in optics, biomedicine, environmental science, materials,
catalysis, energy, and so forth, due to their excellent properties such as versatility,
tunability, and stability. They have attracted enormous interest attributed to their
dramatically tunable physicochemical features. Plasmonic core-shell nanomaterials are
extensively used in surface-enhanced vibrational spectroscopies, in particular,
surface-enhanced Raman spectroscopy (SERS), due to the unique localized surface
plasmon resonance (LSPR) property. This review provides a comprehensive overview of core-shell nanoparticles in the context of fundamental and application aspects of SERS,
and discusses about numerous classes of core-shell nanoparticles with their unique
strategies and functions. Further, herein we also introduce the concept of shell-isolated
nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail, because it overcomes
the long-standing limitations of material and morphology generality encountered in
traditional SERS. We then explain the SERS enhancement mechanism with core-shell
nanoparticles, as well as three generations of SERS hotspots for surface analysis of
materials. To provide a clear view for readers, we summarize various approaches for the
synthesis of core-shell nanoparticles and their applications in SERS, such as
electrochemistry, bioanalysis, food safety, environmental safety, cultural heritage,
materials, catalysis, and energy storage and conversion. Finally, we exemplify about the
future developments in new core-shell nanomaterials with different functionalities for
SERS and other surface-enhanced spectroscopies.
Surface enhanced Raman scattering (SERS)(1–9) was one of the most exciting subjects in surface science for about 10 years after its discovery. It is now widely used as an analytical tool in applied science because of its high sensitivity, although the full consensus on this enhancement mechanism has not yet been reached.
We organize this theoretical discussion around a scheme having three elements: the local field acting on each molecule, the induced molecular dipole and its radiation.
Vibrational spectroscopy is a powerful tool in chemical analysis for determining the kinds and concentrations of molecular species and atoms on a surface, as well as during reactions on a surface. The study of adsorbate vibrational modes leads to a better understanding of their role in energy transfer and dissipation processes and in chemical reactions on surfaces. For adsorbates on metal surfaces, “surface selection rules” hold, which are dependent upon the excitation mechanism and determine the number of discrete vibrations that are to be observed in any spectrum. The selection rules predict the intensities of the vibrational modes, and, in case of metal films, these are expected to be affected either by the thickness of the metal layers or by the diameter of metal particles. In recent years, the awareness of the vibrations of surfaces and adsorbed layers and the interest in the application of vibrational spectroscopy have been growing, mainly because of a steady development of experimental techniques with sufficient resolution and surface sensitivity. The techniques may be classified according to the nature of the exciting probe: photons, electrons, and neutral particles.
Colloidal gold nanocubes coated with a silver nanoshell have been synthesized via the seed mediated growth method. By changing the volume of gold seed and silver nitrate, both the edge length of gold nanocube and the thickness of silver shell could be fine-tuned. The surface-enhanced Raman scattering (SERS) activity of these core-shell structural Au-Ag bimetallic nanocubes has also been investigated by using the rhodamine 6G (R6G) as Raman active probe. It has been found the SERS activity of the silver-coated gold nanocubes greatly depends on their geometry factors. By decreasing the edge length of gold nanocubes or increasing the silver coating thickness, the SERS activity has been greatly enhanced. By comparing with other Raman bands of R6G, the enhancement of the Raman peak corresponding to the C-C-C ring in-plane vibration mode is more sensitive to the geometries of the nanostructure. These improved SERS properties of silver-coated gold nanocubes provide potential application for biologic and chemical sensing based on Raman spectroanalysis.
In recent years, research in electrochemistry has turned increasingly to the combination of data obtained by conventional electrochemical techniques with results derived using a range of ex situ and in situ surface analytical methods(1) (see Table 1). The major reason for this development is that electrochemical techniques inevitably measure the sum of all the processes at the interfaces and, moreover, cannot characterize the molecular species present so that structural information can only be inferred indirectly. In situ methods have a special role to play in this search for molecular specificity, as they are able to characterize the nature and structure of both the electrode and solution sides of the interface; by contrast, ex situ methods can only give information about strongly adsorbed species. In turn, vibrational spectroscopy has a special role among these in situ methods, as vibrational spectra can be used to “finger print” the species present while changes in the spectra of the “telltale” species give information about changes in structure and of the molecular environment of these species.
Raman spectra of nicotinic acid have been measured in aqueous solutions in the pH-range between 1.0 and 12.5. Raman band assignments between 500 cm−1 and 1750 cm−1 are carried out by referring to the compound's solid state Raman spectrum and to spectra of related molecules. Changes of the Raman spectra with solution pH are correlated to the structure change of the nicotinic acid molecule. The surface enhanced Raman spectra (SERS) of nicotinic acid (NA) on a polycrystalline silver electrode are reported in the range 500–1750 cm−1 as a function of pH and applied potential. The results are presented in order to determine the type of adsorbed species (anion, cation, zwitterion). Therefore the SER spectra are compared with the pH dependent solution spectra of NA. These investigations show – in the presence of a non specifically adsorbing electrolyte (0.1 mol l−1 Na2SO4) – the nicotinate anion as the only surface active species detectable within the pH range of 1.4 to 12.5. A detailed analysis of the SER spectra reveals, that the anion exhibits two different molecular orientations towards the metal electrode, a presumably planar alignment dominating at low and medium pH values and a perpendicular one at low proton concentrations. In the intermediate range both orientations exist side by side. The ratio of planar to perpendicular oriented molecules depends on applied potential and increases with growing cathodic polarisation.
The magnitude of the plasma-resonance contribution to surface-enhanced
Raman scattering on silver-island films has been measured with use of an
experimental arrangement in which the resonance can be suppressed. The
measured reduction in Raman scattering is ~ 300, but theoretical
analysis indicates that there may remain an additional unsuppressed
factor <~3. The data show that plasma resonances are not essential
for observation of Raman scattering by molecular monolayers using
standard detection techniques.
Surface-enhanced Raman scattering from molecular monolayers adsorbed at
the aluminum oxide-Ag interface of tunnel junctions has been studied as
a function of the junction parameters and surface roughness. The
surface-plasmon-polariton contribution to surface-enhanced Raman
scattering from this system is quantitatively demonstrated in
experiments on samples fabricated on holographic diffraction gratings.
The existence of a short-range contribution to the surface-enhanced
Raman effect is also shown and compared with the predictions of the
modulated surface dipole mechanism of Jha, Kirtley, and Tsang.
There is an increasing need and challenge for early rapid and accurate
detection, identification, and quantification of chemical, biological,
and energetic hazards in many fields of interest (e.g., medical,
environmental, industrial, and defense applications). Increasingly to
meet these challenges, researchers are turning to interdisciplinary
approaches combining spectroscopy with nanoscale platforms to create
technologies that offer viable and novel solutions for today's sensing
needs. One technology that has gained increasing popularity to meet
these needs is surface enhanced Raman scattering (SERS). SERS is
particularly advantageous as it does not suffer from interferences from
water, requires little to no sample preparation, is robust and can be
used in numerous environments, is relatively insensitive to the
wavelength of excitation employed and produces a narrow-band spectral
signature unique to the molecular vibrations of the analyte. SERS
enhancements (chemical and electromagnetic) are typically observed on
metalized nanoscale roughened surfaces. For ideal SERS sensing, a
commercially available uniform and reproducible nanoscale surface
demonstrating high sensitivity are desirable. Additionally, if these
surfaces can be modified for the selective sensing of hazard materials,
an ideal sensor platform for dynamic in field measurements can be
imagined. In this proceedings paper, preliminary efforts towards the
characterization and application of commercially available next
generation Klarite substrates will be demonstrated. Additionally,
efforts toward chemical modification of these substrates, through
peptide recognition elements, can be used for the targeted sensing of
hazardous materials.
Sensitive, accurate and reliable methods are needed for the detection
and identification of hazardous materials (chemical, biological, and
energetic) in the field. Utilizing such a sensing capability
incorporated into a portable detection system would have wide spread
beneficial impact to the U.S. military and first responder communities.
Surface enhanced Raman scattering (SERS) is increasingly becoming a
reputable technique for the real-time, dynamic detection and
identification of hazard materials. SERS is particularly advantageous as
it does not suffer from interferences from water, requires little to no
sample preparation, is robust and can be used in numerous environments,
is relatively insensitive to the wavelength of excitation employed, and
produces a narrow-band spectral signature unique to the molecular
vibrations of the analyte. We will report on the characterization and
sensing capabilities of these next generation SERS substrates for the
detection of energetic materials (ammonium nitrate, TNT, PETN, and RDX).
Additionally, new efforts producing highly uniform samples, with known
concentrations of energetic materials inkjet printed onto the SERS
sensing surface using a precisely calibrated MicroJet system will be
shown.
There is an increasing need for rapid and accurate detection,
identification, and quantification of chemical, biological, and
energetic hazards in many fields of interest. To meet these challenges,
researchers are combining spectroscopy with nanoscale platforms to
create technologies that offer viable and novel solutions for today's
sensing needs. One technology that has gained increasing popularity to
meet these needs is surface enhanced Raman scattering (SERS). For ideal
SERS sensing, commercially available uniform and reproducible nanoscale
surface demonstrating high sensitivity are desirable. If these surfaces
can be modified for the selective sensing of hazard materials, an ideal
sensor platform for dynamic in field measurements can be imagined. In
this proceedings paper, preliminary efforts towards the characterization
and application of commercially available next generation Klarite
substrates will be demonstrated and efforts towards selective sensing
will be discussed.
Surface plasmon resonance (SPR) can provide a remarkably enhanced electromagetic field around metal surface. It is one of the enhancement models for explaining surface-enhanced Raman scattering (SERS) phonomenon. With the development of SERS theories and techniques, more and more studies referred to the configurations of the optical devices for coupling the excitation and radiation of SERS, including the prism-coupling, waveguide-coupling, and grating-coupling modes. In this review, we will summarize the recent experimental improvements on the surface plasmoncoupled SERS.
The adsorption behavior of p-aminobenzoic acid (PABA) molecules on a silver-coated alumina surface-enhanced Raman scattering (SERS) substrate was investigated. For spotted PABA and PABA in non-polar solvents, the PABA molecule is adsorbed flat on the surface of the SERS substrate. In this orientation, the benzene ring is π-bonded to the substrate, and the molecule is further anchored to the substrate by the binding of the lone pairs of NH2 and COO− groups onto the metal surface. On the other hand, the adsorption behavior of PABA in a polar solvent is greatly influenced by the hydrogen bonding of the amine group with the polar solvent. In this orientation, the molecule is preferentially adsorbed through the COO± and assumes a non-flat orientation on the metal surface.
The SERS spectra of carbonate, hydrocarbonate and several substituted acetic acids absorbed on silver hydrosols are recorded. The greatest enhancement of E′ modes is shown in the spectrum of carbonate, from which the carbonate is deduced to be absorbed in an “end on” configuration, rather than flat on the surface. The spectrum of the hydrocarbonate solution shows the most enhanced bands at about 925 and 620 cm−1, which cannot be explained clearly. All the substituted acids have a most enhanced bands at about 1630 cm−1, revealing that the acids are initially adsorbed in a single bonding state through the carboxyl group. The change in the SERS spectra of the acids with time indicates that a bidentate bridging adsorbed state may be formed after some time.
Surface enhanced Raman scattering of pyridine adsorbed to weakly roughened silver films is studied using a well-characterized extended surface plasmon excitation. We show that the additional increase in the Raman signal associated with the extended surface plasmon is fully accounted for by the enhancement in the average macroscopic field.
The surface enhanced Raman spectra (SERS) of isonicotinic acid adsorbed on a copper electrode were obtained in order to verify their dependence on the type of electrolyte solution, pH and applied potential. The results are discussed considering the most characteristic bands of the species (protonated or nonprotonated) in the ring nitrogen and in the carboxylic group. In specifically adsorbed electrolytes (Cl− and mainly I−) the completely protonated species is more stabilized on the electrode surface than it is in non-specifically adsorbed anions (ClO−4), because of the formation of ion pairs with the coadsorbed halide ions. For more negative potentials, even at low pH values, the spectra are characteristic of the nonprotonated species.
The surface-enhanced Raman scattering (SERS) from thin films formed by p-nitrobenzoic acid (PNBA) adsorbed onto silver island films has been investigated. SERS spectra obtained using low laser powers were very similar to the normal Raman spectra of the sodium salt of PNBA and were characterized by strong bands near 1600 and 1355 cm−1 and by weaker bands near 1395, 1115, and 875 cm−1. The band near 1395 cm−1 was assigned to the symmetric stretching mode of carboxylate groups, indicating that PNBA was adsorbed as a metal salt. When PNBA films were irradiated at high laser powers, a rapid reaction occurred. The bands near 1355 and 1115 cm−1 gradually decreased in intensity and a strong band near 1460 cm−1 and a strong doublet near 1150 cm−1 gradually appeared. The band near 1460 cm−1 and the doublet near 1150 cm−1 were attributed to azodibenzoate formed by the reductive coupling of PNBA molecules at the silver surface during laser irradiation. When adsorbed PNBA films were irradiated at low laser powers, the reaction still occurred but at a much lower rate. Reduction of PNBA was probably thermally induced but a photochemical mechanism may also be possible.
Surface-enhanced Raman spectra (SERS) of pyridine adsorbed on copper, silver, and gold films vapor deposited on low temperature substrates are reported. Similar spectra were also obtained on a sputter-cleaned silver single crystal. Excitation spectra (450–750 nm) for all three metals revealed an overall increase in SERS activity at longer wavelengths, the relative increase being greater for copper and gold than silver. A broad excitation maximum near 2 eV was observed for the 1006 cm−1 pyridine SERS signal on silver. A lesser-defined maximum was revealed for copper in the same general vicinity (1.7–2.0 eV) while a broad onset extending below 1.7 eV was observed for gold. Temperature studies (15–300 K) indicate that the observed SERS originate from molecular pyridine chemisorbed to the metal surfaces. In the cases of copper and gold, SERS were also observed from samples maintained at room temperature in vacuum. The intensities of the SERS signals were proportional to incident (cw) laser power at low power densities (
The most recently developed diagnostic technique in metal-electrolyte and metal-gas interfaces adapts spontaneous Raman scattering and nonlinear optical generation, techniques normally applied to bulk media, to surface science investigation. For certain metallic surfaces, an enormous increase exists in the Raman (as much as 106 to 108 times) and nonlinear optical signals resulting from submonolayer coverage of molecular adsorbates at the interface. Spontaneous Raman scattering and nonlinear optical scattering are well developed in both theory and practice for the analysis of molecular structure and concentration in bulk media. Instrumentation to generate and detect these inelastically scattered signals is readily available and is adequate for adaption to surface science. However, the mechanism (or mechanisms) giving rise to such a large enhancement at the interfaces is still being actively researched and remains controversial. Theoretical and experimental investigations related to the underlying physics of this enhancement and the application of such surface enhancement as a vibrational probe for adsorbates on the metal surface have been labeled “surface-enhanced Raman scattering” (SERS) and “surface-enhanced nonlinear optics”. Soon after the recognition that molecules adsorbed onto metal electrodes under certain conditions exhibit an anomalously large Raman scattering efficiency,1–3 it became evident that such a phenomenon makes possible an in situ diagnostic probe for detailed and unique vibrational signatures of adsorbates in the ambient phase (electrolyte and atmospheric gas surroundings). Optical spectroscopy in the visible range has a much higher energy resolution (e.g., 0. I cm-I) than is presently available in electron energy loss spectroscopy (EELS), as well as the capability to measure much lower frequency modes (e.g., as low as 5 cm−1) than is possible in infrared spectroscopy. Perhaps the most significant attribute of SERS and surface-enhanced nonlinear optical scattering is that the surrounding media in front of the interface (e.g., several meters of gas and several centimeters of liquid) do not introduce optical loss or overwhelmingly large signals. The recognition that SERS is capable of performing vibrational spectroscopy with this resolution, frequency range, and in such dense surroundings has therefore brought an explosion of activity to the field since 1977.
The combination of surface-enhanced Raman spectroscopy (SERS) with near-infrared (NIR) excitation should be of considerable benefit for trace detection of biological and environmental samples. Enhancement factors for SERS of p-aminobenzoic acid (PABA) on silver/alumina, silver island, gold/alumina and gold island surfaces were determined using unenhanced FT-Raman spectra of PABA on glass and glass/alumina surfaces as references. The silver/alumina substrate provided the greatest enhancement factor, followed by silver island, gold/alumina and gold island substrates, respectively. An enhancement factor in excess of 106 for PABA on a silver/alumina substrate compares favourably with literature reports of similar systems at shorter wavelengths.
The surface enhanced Raman scattering (SERS) of benzoic acid adsorbed on silver sol is presented. A complete assignment of the SER spectrum is given and the nature, the structure and the orientation of the chemisorbed species are discussed. A comparison is made with related results available in literature.
A metal overlayer FT-IR-ATR technique that is found to substantially enhance the signal-to-noise ratio of monolayer and submonolayer film spectra is described. Experimental evidence of the enhancement is given for, as examples, a deposited poly(vinyl acetate) film of 0.5 or 5 nm and a thin film of adsorbed protein human serum albumin, estimated to be of the order of 5 nm. The evidence is supported by simulations showing the expected trends in ΔR as a function of thickness of the air gap between the film sample and the metal overlayer, the thickness of the film sample itself, and electric field diagrams illustrating that polarization dependence and enhancement effects can result from a correct choice of optical configuration. The metals investigated include gold and aluminum. The practical aspects and limitations of this technique are also discussed.
Surface-enhanced Raman scattering (SERS) by films of polystyrene adsorbed onto silver island films was investigated. Films that were only a few tens of angstroms in thickness degraded rapidly during laser irradiation to form graphite-like species at the silver surface. However, no degradation was observed while Raman spectra of the solid polymer were obtained, indicating that the graphitization was probably induced by laser heating of the substrate and catalyzed by silver. For thin films of polystyrene, the rate of graphitization was high and was proportional to laser power. However, the degradation reaction was inhibited for thick films or for thin films overcoated with thick films of a second polymer. The Raman spectra were similar for all films thicker than approximately a hundred angstroms, even those overcoated with a thick film of a second polymer having a large Raman scattering cross section, indicating that most of the observed scattering originated from polymer molecules within a few tens of angstroms of the silver surface. It was concluded that SERS can be used to probe the molecular structure of polymer/metal interfaces without interference by scattering from the bulk of the polymer.
Using the excitation of surface electromagnetic waves in the Kretschmann configuration, a fairly large field is produced at the metal-air interface for infrared frequency. The results indicate the possibility of enhanced infrared absorption of adsorbed molecules by thin metal films.
Surface enhanced Raman scattering (SERS) of crystal violet adsorved on continuous and on island films of silver has been measured with the attenuated total reflection (ATR) method, and the enhanced Raman intensities were compared with surface plasma resonance (SPR) excited on the silver films, also measured by the ATR method. A linear relation between SPR and SERS (independent of its form) was found, which can be explained simply by the power transformed from the excitation light to the SPR.
Photoluminescence of silver in the forms of bulk, thin film, powder, and colloids was found by irradiating the samples with UV laser beams. Two mechanisms, i.e., the surface plasma resonance and the interband transitions are attributed to the observed spectra.
The Raman spectra of 4-pyridine-carboxaldehyde-doped Al-Al2O3-Ag tunnel junctions evaporated onto CaF2 films and optical diffraction gratings show that the molecular Raman scattering is strongly enhanced under conditions which permit the direct excitation of the surface plasmon modes of Ag. This suggests that the surface plasmon is an intermediate state in surface-enhanced Raman scattering.
Using a general formulation we show that by using surface electromagnetic waves in an attenuated-total-reflection prism configuration it should be possible to enhance the intensity of Raman scattering by a thin overlayer on a Ag surface by two orders of magnitude and that the use of surface electromagnetic waves may in fact make it possible to observe coherent anti-Stokes Raman scattering by the overlayer.
Anomalously strong Raman spectra have been obtained from molecular monolayers adsorbed on the insulator in metal-insulator-metal tunnel junctions. We show unambiguously that Raman spectroscopy can readily detect molecular monolayers and consider the effects of surface roughness, the molecule-metal interface and the metal on our results.
It is now well established that surface roughness on a sub-microscopic scale plays an Important role in the enhancement of the Raman scattering (RS) by molecules adsorbed on metals.1–4 Moreover, recent work indicates that the optical absorption and luminescence by molecules adsorbed at a rough metal surface can also be enhanced.5–8
The unusually large Raman signals originating from molecules adsorbed on electrode surfaces are postulated to arise from resonance enhancement by electronic processes at the rough metal-eletrolyte surface. By modelling this transition region between metal and eletrolyte with a collection of metallic spheres surrounded by adsorbate and ambient medium on top of a flat metal substrate, satisfactory agreement is obtained between measured and calculated spectra of both ΔR/R and the excitation function of the Raman signals.
We present a calculation of the angular distribution of surface-plasmon radiation from a rough surface. The radiated fields are calculated to first order in the surface-roughness height δ, and the results are compared with our earlier measurements. The present result is found to compare more favorably with experiment than a previous calculation by Kretschmann. The strength of the "backscattering" of surface plasmons from small surface features is determined and used to discuss certain features of the experimental results.
The inelastic tunneling of electrons in a metal-insulator-metal junction has been shown to be a spectroscopic method for studying the vibrational modes of the whole system. In the present paper we consider the possibility of deducing precise information from this spectroscopy. The low-voltage part of the spectrum (i.e., the d2I/dV2-vs-V characteristic) gives information about the phonons of the electrodes. The phonon density, which is deduced for a Mg electrode, is critically compared with the density deduced from neutron scattering. The range of this phonon probe is then studied by tunneling into multilayer electrodes. The 40-90-meV range of the characteristic of a Mg-Pb junction exhibits a specific structure due to the lattice vibrations of the insulator. This structure is compared with the infrared spectrum and the phonon density of states of MgO, as well as with a theoretical calculation of the tunneling current in the transfer-Hamiltonian formalism. From the fit obtained, it is deduced that the 30-Å-thick insulator, grown on Mg, is an oxide, in contrast with the insulator grown on Al, which was previously deduced to be a hydroxide. At higher energies (100-500 meV), the vibrational spectrum of molecules contained in the insulator region is observed. The identification of the lines is shown to be accurate and it gives precise information on these molecules, especially about their chemical binding with the insulator. This last point could be important in the future for studying the problem of adsorption on solid surfaces.
Anomalous enhancement of the Raman intensity from pyridine and cyanide adsorbed onto metals has been investigated with particular attention to the role of surface chemistry. We have correlated the current-voltage characteristics of the Ag/cyanide system with the appearance and disappearance of the enhancement. We have chemically modified Au by sub-monolayer deposits of Ag to provide sites through which pyridine Raman scattering is enhanced under excitation conditions that normally show no enhancement. Comparison of cyanide etfects on Ag electroreflectance with the enhanced cyanide Raman scattering has shown that two distinctly different forms of adsorbed cyanide exist, only one of which is giant Raman active. We conclude from these experiments that a particular chemical bond between the molecule and the substrate is the most important prerequisite for enhanced scattering.
A method is given to determine accurately the optical constants and the thickness of thin films when the real and the imaginary part of the dielectric constants obey the condition ɛ r <- 1, ɛ i ¦ɛ r ¦. The method makes use of the possibility to excite surface plasma waves with the help of the inhomogeneous light wave obtained by total reflexion. The accuracy of the method is pointed out. As an example the optical constants of silver foils in the wavelength interval 4000 to 6000 Å are determined.
The giant RS by pyridine and CN− on Ag is accompanied by a strong RS continuum which is attributed to inelastic light scattering by charge carrier-excitations. The enhanced RS by the adsorbed molecules and by the charge carrier-excitations are attributed to surface roughness enhanced EM fields at the metal surface resulting from the excitation of transverse collective electron-excitations and surface-EM modes, and to surface roughness-induced radiative-excitation and radiative-recombination of particle-hole pairs.
The strong dependence of the surface Raman intensity on the exciting frequency and on the angle of incidence for pyridine molecules adsorbed on Au, Ag and Cu electrodes after a weak oxidation/reduction cycle is evidence for a surface plasmon enhanced Raman scattering (SPERS).
We analyze several theoretical models which have recently been proposed in an attempt to explain the anomalously high Raman intensity from molecules adsorbed onto metal surfaces. We classify the theories according to their primary mechanism, pointing out similarities and differences, some which have not previously been recognized in print. We present a critical analysis of the conceptual basis and an outline of the experimental implications of each theory. We conclude that several theories have serious problems in their associated assumptions, and that other theories have been brought into question by recent experimental results. We propose additional experiments which would help clarify the situation.