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Quantitative PEEM and Raman Study of Nanorough Au SERS-Active Substrates for Molecular Sensing Applications
... Photoemission electron microscopy (PEEM), a high-resolution near-field mapping technique (Douillard et al. 2007;Awada et al. 2012), can be used to collect high-contrast electronic images reflecting the distribution of hot spots on a sample surface at a subwavelength scale. Recently, this technique was used for studying the variation in hot spot density per unit area of an Au film as a function of the process conditions (Taugeron et al. 2023). We were able to optimize nanorough Au film substrates fabricated by physical vapor deposition (PVD) and show, for the first time of our knowledge, that the near field optical responses of our bare fresh surfaces are quantitatively correlated with their far field Raman diffusion signature when they are used as molecular sensors. ...
... As a standard target molecule, thiophenol in alcoholic solution diluted at 10 -6 M and 10 -8 M is used. This molecule is commonly used for SERS analysis, because it adsorbs easily on the gold surface via its thiol function to form, on a flat surface when the concentration is high enough, a homogeneous layer after rinsing (Noh et al. 2010;Taugeron et al. 2023). For 10 -8 M thiophenol solutions, we will be in more drastic detection conditions, since it will be impossible to cover the entire active surface of our substrates with a homogeneous and dense layer of molecules. ...
... Our home-made substrates are fabricated by physical vapor deposition (thermal evaporation) of Au. The fabrication method was recently reported (Taugeron et al. 2023). Briefly, the samples are prepared by a reproducible two-step thermal evaporation process: a Au layer of approximately 100 nm is first deposited on a Si wafer (the film thickness being controlled by a quartz microbalance system) with a rate of 0.1 nm/s at a pressure of 5.10 -6 mbar. ...
Surface enhanced Raman spectroscopy (SERS) is a powerful non-invasive technique to detect and identify molecule traces. The accurate identification of molecules is based on the detection of distinctive vibrational modes characteristic of a molecule adsorbed onto the surface. We investigated the detection performances of three commercial SERS substrates: nanostructured Au supports from Hamamatsu, Premium Ag–Au supports from SERSitive, and RAM–SERS–Au from Ocean Insight, which were tested with solutions of thiophenol (C6H6S) at 10–6 M and 10–8 M concentration. SERS measurements were performed systematically with 633 and 785 nm excitation wavelengths and Raman mappings were recorded randomly on the surfaces. The spectral quality (baseline intensity and signal-to-noise ratio), the thermal stability under laser illumination, and the Raman intensity distribution of the Hamamatsu substrate and our own fabricated gold substrate were discussed for the detection of 10–8 M thiophenol molecules. The detection of crystal violet (CV), a toxic dye, is demonstrated at 5.10–9 M.
Transparent conductors are essential elements in an array of optoelectronic devices. The most commonly used transparent conductor – indium tin oxide (ITO) suffers from issues including poor mechanical flexibility, rising cost, and the need for annealing to achieve high conductivity. Consequently, there has been intensive research effort in developing ITO‐free transparent conductors over the recent years. This article gives a comprehensive review on the development of an important ITO‐free transparent conductor, that is based on thin metal films. It starts with the background knowledge of material selection for thin‐metal‐film‐based transparent conductors and then surveys various techniques to fabricate high‐quality thin metal films. Then, it introduces the spectroscopic ellipsometry method for characterizing thin metal films with high accuracy, and discusses the optical design procedure for optimizing transmittance through thin‐metal‐film‐based conductors. The review also summarizes diverse applications of thin‐metal‐film‐based transparent conductors, ranging from solar cells and organic light emitting diodes, to optical spectrum filters, low‐emissivity windows, and transparent electromagnetic interference coatings.
In order to expand the spectral capabilities of the SERS method, Ag–Cu alloy nanoparticle arrays were studied. Arrays of Ag–Cu nanoparticles were formed by vacuum thermal evaporation and condensation on an unheated substrate followed by heat treatment at 230 °C. A higher Raman scattering enhancement in the red region of the spectrum when using an array of nanoparticles of the Ag–Cu eutectic system was found. This enhancement is comparable to the one of pure silver particle array in the blue region of the spectrum. Transmission electron microscopy study has shown that the feature of the Ag–Cu array is that many particles are composite: one part consists of copper, and the other part is made up of silver. These parts have a perfect (possibly heteroepitaxial) boundary. It is believed that the localized surface plasmon resonance excited in the copper part by red light can be transmitted without any loss into the silver part, while the one excited in the silver part by blue light, quickly fades out in the copper part, because blue light has a higher frequency than the copper plasma frequency. From the point of view of SERS applications, the use of Ag–Cu particle arrays allows extending the advantage of silver to the red region of the spectrum.
Surface enhanced Raman spectroscopy (SERS) has been intensively investigated during the past decades for its enormous electromagnetic field enhancement near the nanoscale metallic surfaces. Chemical enhancement of SERS, however, remains rather elusive despite intensive research efforts, mainly due to the relatively complex enhancing factors and inconsistent experimental results. To study details of chemical enhancement mechanism, we prepared various low dimensional semiconductor substrates such as ZnO and GaN that were fabricated via metal organic chemical vapor deposition process. We used three kinds of molecules (4-MPY, 4-MBA, 4-ATP) as analytes to measure SERS spectra under non-plasmonic conditions to understand charge transfer mechanisms between a substrate and analyte molecules leading to chemical enhancement. We observed that there is a preferential route for charge transfer responsible for chemical enhancement, that is, there exists a dominant enhancement process in non-plasmonic SERS. To further confirm our idea of charge transfer mechanism, we used a combination of 2-dimensional transition metal dichalcogenide substrates and analyte molecules. We also observed significant enhancement of Raman signal from molecules adsorbed on 2-dimensional transition metal dichalcogenide surface that is completely consistent with our previous results. We also discuss crucial factors for increasing enhancement factors for chemical enhancement without involving plasmonic resonance.
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.
We developed a method to fabricate sensitive and stable substrates that can be used in routine surface-enhanced Raman scattering (SERS) spectral measurements. The substrates were fabricated by immobilization of Ag colloidal particles on a glass plate coated with poly(4-vinyl pyridine) (P4VP), adsorption of molecules on the immobilized colloidal particles, and second immobilization. In the second step, we adsorbed weakly adsorbed molecules such as aniline instead of target molecules. We purified pristine silver sols by centrifuging to increase the surface charge of the colloidal particles and optimized Ag colloidal particle diameter to achieve high Raman signal enhancement from their dimers. When purified silver sols were used in the immobilizations, dimers were predominantly formed with very few trimers. When target molecular solutions were dropped in arrays on the substrate prepared by adsorbing aniline and dried, a good SERS spectrum was observed from each spot without any peaks of aniline and P4VP. The average enhancement was found to be 1.0 × 10⁷ and the enhancement of the dimers was ∼1.6 × 10⁷. The latter corresponds to the enhancement outside the hotspot regions, since target molecules could not be adsorbed at the junctions after the formation of dimers. About one hundred samples could be tested on the substrate fabricated on a cover glass when target molecular solutions were dropped by using a micropipette. Since dropping and drying are very simple and easy processes, and many samples could be tested on a substrate, our substrates can be used in routine SERS spectral measurements and assays like bioassays.
Electromagnetic field enhancement (FE) effects occurring in thin gold films 3-12-nm are investigated with two-photon photoluminescence (TPL) and Raman scanning optical microscopies. The samples are characterized using scanning electron microscopy images and linear optical spectroscopy. TPL images exhibit a strong increase in the level of TPL signals for films thicknesses 3–8-nm, near the percolation threshold. For some thicknesses, TPL measurements reveal super-cubic dependences on the incident power. We ascribe this feature to the occurrence of very strongly localized and enhanced electromagnetic fields due to multiple light scattering in random nanostructures that might eventually lead to white-light generation. Raman images exhibit increasing Raman signals when decreasing the film thickness from 12 to 6-nm and decreasing signal for the 3-nm-film. This feature correlates with the TPL observations indicating that highest FE is to be expected near the percolation threshold.
Ultra-thin films with low surface roughness that support surface plasmon-polaritons in the infra-red and visible ranges are needed in order to improve the performance of devices based on the manipulation of plasmon propagation. Increasing amount of efforts is made in order not only to improve the quality of the deposited layers but also to diminish their thickness and to find new materials that could be used in this field. In this review, we consider various thin films used in the field of plasmonics and metamaterials in the visible and IR range. We focus our presentation on technological issues of their deposition and reported characterization of film plasmonic performance.
Aligned silver nanorod (AgNR) array films were fabricated by oblique thermal evaporation. The substrate temperature during evaporation was varied from 10 to 100 °C using a home-built water cooling system. Deposition angle and substrate temperature were found to be the most important parameters for the morphology of fabricated films. Especially, it was found that there exists a critical temperature at ~90 °C for the formation of the AgNR array. The highest enhancement factor of the surface-enhanced Raman scattering (SERS), observed in the Ag films coated with benzenethiol monolayer, was ~6 × 10(7). Hot spots, excited in narrow gaps between nanorods, were attributed to the huge enhancement factor by our finite-difference time-domain (FDTD) simulation reflecting the real morphology.
Localization of optical excitations within subwavelength areas of a random metal-dielectric film is observed using near-field scanning optical microscopy. This effect is attributed to Anderson localization of surface plasmon modes in a semicontinuous metal film. The localized modes are seen as giant fluctuations of local electric fields spatially concentrated in “hot” spots, where the fields are much greater than the applied field. The local near-field spectra consisting of strong resonance peaks are detected and shown to depend markedly on the sample site probed. The observed spectral peaks correspond to localized modes of random metal-dielectric films.
A novel laboratory experiment was successfully implemented for undergraduate and graduate students in physical chemistry and nanotechnology. The main goal of the experiment was to rigorously determine the surface-enhanced Raman scattering (SERS)-based sensing capabilities of colloidal silver nanoparticles (AgNPs). These were quantified by estimating the analytical enhancement factor (AEF) and surface enhancement factor (SEF) of the AgNPs, that is, the most important values for characterizing the SERS effect. SERS is an embodiment of Raman spectroscopy that currently finds numerous cutting-edge applications. To achieve this, students synthesized a Creighton colloid and characterized its optical properties by UV-vis absorption spectrophotometry. The AEF and SEF values were then estimated from the measured Raman, SERS, and fluorescence emission spectra. of a test probe, rhodamine 6G, adsorbed on the colloidal AgNPs. This laboratory experiment introduced the students to the fundamentals of SERS spectroscopy and implicitly to concepts related to light scattering, surface chemistry, and resonance effects. Furthermore, students acquired new instrumental and nanotechnology-related skills that will assist them in technologically demanding careers.
Metallic nanoparticles (NPs) have received recently considerable interest of photonic and photovoltaic communities. In this work, we report the optoelectronic properties of gold NPs (Au-NPs) obtained by depositing very thin gold layers on glass substrates through thermal evaporation electron-beam assisted process. The effect of mass thickness of the layer was evaluated. The polycrystalline Au-NPs, with grain sizes of 14 and 19 nm tend to be elongated in one direction as the mass thickness increase. A 2 nm layer deposited at 250°C led to the formation of Au-NPs with 10-20 nm average size, obtained by SEM images, while for a 5 nm layer the wide size elongates from 25 to 150 nm with a mean at 75 nm. In the near infrared region was observed an absorption enhancement of amorphous silicon films deposited onto the Au-NPs layers with a corresponding increase in the PL peak for the same wavelength region.
Local field distributions in random metal-dielectric films near a percolation threshold are experimentally studied using scanning near-field optical microscopy (SNOM). The surface-plasmon oscillations in such percolation films are localized in small nanometer-scale areas, “hot spots,” where the local fields are much larger than the field of an incident electromagnetic wave. The spatial positions of the hot spots vary with the wavelength and polarization of the incident beam. Local near-field spectroscopy of the hot spots is performed using our SNOM. It is shown that the resonance quality-factor of hot spots increases from the visible to the infrared. Giant local optical activity associated with chiral plasmon modes has been obtained. The hot spot’s large local fields may result in local, frequency and spatially selective photomodification of percolation films.
A wide variety of methods have been used to compute percolation thresholds. In lattice percolation, the most powerful of these methods consists of microcanonical simulations using the union-find algorithm to efficiently determine the connected clusters, and (in two dimensions) using exact values from conformal field theory for the probability, at the phase transition, that various kinds of wrapping clusters exist on the torus. We apply this approach to percolation in continuum models, finding overlaps between objects with real-valued positions and orientations. In particular, we find precise values of the percolation transition for disks, squares, rotated squares, and rotated sticks in two dimensions and confirm that these transitions behave as conformal field theory predicts. The running time and memory use of our algorithm are essentially linear as a function of the number of objects at criticality.
The heterogeneous character of thin gold films prepared by thermal evaporation and the dependence of this heterogeneity on the rate of their deposition must be considered when exploiting their optical properties for biosensor purposes. For instance, the performance of thin gold films for surface plasmon resonance (SPR) biosensors may drastically be degraded if care is not taken to prepare a film with a high fraction of gold (>95%). We use three different models to interpret the SPR response of gold films prepared by thermal evaporation. We show that the interpretation of the SPR curves requires considering both a global heterogeneity of the gold films and a surface roughness. Our conclusions are further corroborated by scanning surface plasmon microscope (SSPM) images of these thin gold films.
A new method of exciting nonradiative surface plasma waves (SPW) on smooth surfaces, causing also a new phenomena in total reflexion, is described. Since the phase velocity of the SPW at a metal-vacuum surface is smaller than the velocity of light in vacuum, these waves cannot be excited by light striking the surface, provided that this is perfectly smooth.However, if a prism is brought near to the metal vacuum-interface, the SPW can be excited optically by the evanescent wave present in total reflection. The excitation is seen as a strong decrease in reflection for the transverse magnetic light and for a special angle of incidence. The method allows of an accurate evaluation of the dispersion of these waves. The experimental results on a silver-vacuum surface are compared with the theory of metal optics and are found to agree within the errors of the optical constants.
This review article presents a general view of the recent progress in the fast developing area of surface-enhanced Raman scattering spectroscopy as an analytical tool for the detection and identification of molecular species in very small concentrations, with a particular focus on potential applications in the biomedical area. We start with a brief overview of the relevant concepts related to the choice of plasmonic nanostructures for the design of suitable substrates, their implementation into more complex materials that allow generalization of the method and detection of a wide variety of (bio)molecules and the strategies that can be used for both direct and indirect sensing. In relation to indirect sensing, we devote the final section to a description of SERS-encoded particles, which have found wide application in biomedicine (among other fields), since they are expected to face challenges such as multiplexing and high-throughput screening.
Aggregates of Au nanoparticles have been extremely easily obtained on glass substrates by
physical sputtering under primary vacuum. With such a protocol, we demonstrate that it
is possible to control the surface plasmon band absorption. Surface enhanced
Raman spectroscopy (SERS) experiments were performed with methylene blue, zinc
octacarboxyphthalocyanine, 4-aminothiophenol and cysteamine. The correlation between
the absorption band and the wavelength giving the highest SERS intensity is clearly
observed for methylene blue, in accordance with the electromagnetic enhancement theory.
For the other molecules, effects of the chemical enhancement are also observed. In
addition, we noticed a strong influence of the nature of the adsorbed molecule
on the enhancement factor for a given wavelength. The origin of this feature
is discussed in terms of resonant effects or multipolar surface plasmon modes.
A scale-invariant theory of Raman scattering of light by fractal clusters is developed. The enhancement factor GRS of Raman scattering is shown to scale in terms of a properly chosen spectral variable X. The critical indices of the enhancement factor are found to be determined by the optical spectral dimension of the fractal. Numerical modeling is carried out and shown to support the analytical results obtained. The theory, which does not contain any adjustable parameters, agrees well with experimental data on surface-enhanced Raman scattering over a wide spectral range.
Highly localized, laser-excited optical modes of silver colloid fractal clusters were observed using photon scanning tunneling microscopy. The spatial distribution of the modes shows the frequency and polarization selectivity suggested by numerical simulations. The results verify the main concepts of the recently developed resonant optical theory of fractal objects.
Surface enhanced Raman spectroscopy (SERS) is a powerful non-invasive technique to detect and identify molecule traces. The accurate identification of molecules is based on the detection of distinctive vibrational modes characteristic of a molecule adsorbed onto the surface. We investigated the detection performances of three commercial SERS substrates: nanostructured Au supports from Hamamatsu, Premium Ag–Au supports from SERSitive, and RAM–SERS–Au from Ocean Insight, which were tested with solutions of thiophenol (C6H6S) at 10–6 M and 10–8 M concentration. SERS measurements were performed systematically with 633 and 785 nm excitation wavelengths and Raman mappings were recorded randomly on the surfaces. The spectral quality (baseline intensity and signal-to-noise ratio), the thermal stability under laser illumination, and the Raman intensity distribution of the Hamamatsu substrate and our own fabricated gold substrate were discussed for the detection of 10–8 M thiophenol molecules. The detection of crystal violet (CV), a toxic dye, is demonstrated at 5.10–9 M.
In this work, TiO2 nanotubes (NT) and nanograss (NG) at the edges of nanotubes obtained by electrochemical anodizing followed by the deposition of plasmonic silver (Ag, 30μg/cm²) nanoparticles were investigated as substrates for surface enhanced Raman scattering (SERS) on methylene blue molecules. The resulting nanotubes have an inner diameter of about 40–80 nm and a wall thickness of ∼10 nm. Synthesis parameters were worked out for the formation of nanograss on the edges of TiO2 nanotubes. The deposition of an active silver layer with respect to surface plasmon resonance on the TiO2 nanostructures makes it possible to use this composite as efficient SERS substrates. It was found that the proposed SERS systems allow to detect the Raman spectrum of methylene blue (MB) at concentrations down to 10⁻¹² M. The values of the analytical enhancement factor for TiO2 NT and NG at the edges of nanotubes were estimated with respect to different reference materials.
SERS substrate containing SERS-active array of Ag, Au and Ag50Au50 nanoparticles with normal size distribution, a mirror layer and a separating SiO2 layer was optimized. A layer of amorphous carbon was used as the object of study. It is shown that such a structure enhances the Raman signal both due to the localized surface plasmon resonance and due to interference. The average size of nanoparticles, the reflection coefficient of mirror layer, and the thickness of SiO2 layer influence the amplification of the Raman signal. To provide amplification the thickness of SiO2 layer should be calculated for the appropriate wavelength in order to get constructive interference. It is revealed that additional amplification of the Raman signal occurs if the mirror layer as well as the array of particles is made of plasmon metal and thickness of separating SiO2 layer is quite small (up to 30 nm).
The semicontinuous gold film, enabling various electronic applications including development of surface-enhanced Raman scattering (SERS) substrate, is investigated using conductive atomic force microscopy (CAFM) and Kelvin probe force microscopy (KPFM) to reveal and investigate local electronic characteristics potentially associated with SERS generation of the film material. Although the gold film fully covers the underlying silicon surface, CAFM results reveal that local conductivity of the film is not continuous with insulating nanoislands appearing throughout the surface due to incomplete film percolation. Our analysis also suggests the two-step photo-induced charge transfer (CT) play the dominant role in the enhancement of SERS intensity with strong contribution from free electrons of the silicon support. Silicon-to-gold charge transport is illustrated by KPFM results showing that Fermi level of the gold film is slightly inhomogeneous and far below the silicon conduction band. We propose that inhomogeneity of the film workfunction affecting chemical charge transfer between gold and Raman probe molecule is associated with the SERS intensity varying across the surface. These findings provide deeper understanding of charge transfer mechanism for SERS which can help in design and development of the semicontinuous gold film-based SERS substrate and other electronic applications.
Effective surface-enhanced Raman scattering (SERS)-active substrates from gold nanoparticle and gold nanohole arrays were successfully fabricated through electron beam lithography with precise computer-aided control of the unit size and intergap distance. Their SERS performance was evaluated using 4-mercaptobenzoic acid (4-MBA). These gold arrays yielded strong SERS signals under 785 nm laser excitation. The enhancement factors for 4-MBA molecules on the prepared gold nanoparticle and nanohole arrays maxed at 1.08 × 10⁷ and 8.61 × 10⁶, respectively. The observed increase in SERS enhancement was attributed to the localized surface plasmon resonance (LSPR) wavelength shifting toward the near-infrared regime when the gold nanohole diameter increased, in agreement with the theoretical prediction in this study. The contribution of LSPR to the Raman enhancement from nanohole arrays deposited on fluorine-doped tin oxide glass was elucidated by comparing SERS and transmission spectra. This simple fabrication procedure, which entails employing electron beam lithography and the controllability of the intergap distance, suggests highly promising uses of nanohole arrays as functional components in sensing and photonic devices.
Probing nanooptical near-fields is a major challenge in plasmonics. Here, we demonstrate an experimental method utilizing ultrafast photoemission from plasmonic nanostructures that is capable of probing the maximum nanoplasmonic field enhancement in any metallic surface environment. Directly measured field enhancement values for various samples are in good agreement with detailed finite-difference time-domain simulations. These results establish ultrafast plasmonic photoelectrons as versatile probes for nanoplasmonic near-fields.
Nanostructured gold substrates provide chemically stable, signal-enhancing substrates for the sensitive detection of a variety of compounds through surface-enhanced Raman spectroscopy (SERS). Recent developments in advanced fabrication methods have enabled the manufacture of SERS substrates with repeatable surface nanostructures that provide reproducible quantitative analysis, historically a weakness of the SERS technique. Here, we describe the novel use of gold-sputtered Blu-ray disc surfaces as SERS substrates. The unique surface features and composition of the Blu-ray disc recording surface lead to the formation of gold nano-islands and nanogaps following simple gold sputtering, without any background peaks from the substrate. The SERS performance of this substrate is strong and reproducible with an enhancement factor (EF) of 103 for melamine. A limit of detection (LOD) for this compound of 70 ppb and average reproducibility of ±12 % were achieved. Gold-sputtered Blu-ray discs thus offer an excellent alternative to more exotic gold SERS substrates prepared by advanced, time-consuming and expensive methods.
Graphical abstract
AFM 3D images of 1-μm2 sections of uncoated and gold-sputtered recordable Blu-ray disc (BD-R) surfaces and the SERS signal obtained on the gold-sputtered surface for a 1000 ppm aqueous solution of melamine.
The need for a dedicated spectroscopic technique with nanoscale resolution to characterize SERS substrates pushed us to develop a proof of concept of a functionalized tip - surface enhanced Raman scattering (FTERS) technique. We have been able to map hot spots on semi-continuous gold films; in order to validate our approach we compare our results with photoemission electron microscopy (PEEM) data, the complementary electron microscopy tool to map hot spots on random metallic surfaces. Enhanced Raman intensity maps at high spatial resolution reveal the localisation of hotspots at gaps for many neighboring nanostructures. Finally, we compare our findings with theoretical simulations of the enhancement factor distribution; which confirms a dimer effect as the dominant origin of hot spots.
Plasmonic nanostructures enable light to be concentrated into nanoscale 'hotspots', wherein the intensity of light can be enhanced by orders of magnitude. This plasmonic enhancement significantly boosts the efficiency of nanoscale light-matter interactions, enabling unique linear and nonlinear optical applications. Large enhancements are often observed within narrow gaps or at sharp tips, as predicted by the classical electromagnetic theory. Only recently has it become appreciated that quantum mechanical effects could emerge as the feature size approaches atomic length-scale. Here we experimentally demonstrate, through observations of surface-enhanced Raman scattering, that the emergence of electron tunnelling at optical frequencies limits the maximum achievable plasmonic enhancement. Such quantum mechanical effects are revealed for metallic nanostructures with gap-widths in the single-digit angstrom range by correlating each structure with its optical properties. This work furthers our understanding of quantum mechanical effects in plasmonic systems and could enable future applications of quantum plasmonics.
The fabrication of nanostructured substrates with precisely controlled geometries and arrangements plays an important role in studies of surface-enhanced Raman scattering (SERS). Here, we present two processes based on electron-beam lithography to fabricate gold nanostructures for SERS. One process involves making use of metal lift-off and the other involves the use of the plasma etching. These two processes allow the successful fabrication of gold nanostructures with various kinds of geometrical shapes and different periodic arrangements. 4-mercaptopyridine (4-MPy) and Rhodamine 6G (R6G) molecules are used to probe SERS signals on the nanostructures. The SERS investigations on the nanostructured substrates demonstrate that the gold nanostructured substrates have resulted in large SERS enhancement, which is highly dependent on the geometrical shapes and arrangements of the gold nanostructures.
This paper describes the fabrication of gold nanopillar and nanorod arrays and theoretical calculations of electromagnetic fields (EMFs) around ordered arrangements of these nanostructures. The EMFs of both single nanopillars and di-mers of nanopillars - having nanoscale gaps between the two adjacent nanopillars forming the di-mers - are simulated in this work by employing the Finite Difference Time Domain (FDTD) method. In the case of simulations for di-mers of nanopillars, the nano-scale gaps between the nanopillars are varied between 5 nm and 20 nm and calculations of the electromagnetic fields in the vicinity of the nanopillars and in the gaps between the nanopillars were carried out. Fabrication of gold nanopillars in a controlled manner for forming SERS substrates involves focused ion beam (FIB) milling. The nanostructures were fabricated on gold-coated silica, mica, and quartz planar substrates as well as on gold-coated tips of four mode and multimode silica optical fibers.
We present an introduction to surface-enhanced Raman scattering (SERS) which reviews the basic experimental facts and the essential features of the mechanisms which have been proposed to account for the observations. We then review very recent fundamental developments which include: SERS from single particles and single molecules; SERS from fractal clusters and surfaces; and new insights into the chemical enhancement mechanism of SERS.
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.
In this paper, an experimental study of hot spots in gold/dielectric films using photoemission electron microscopy is reported. This technique allows a characterization of the statistical optical properties with unprecedented accuracy in the 800- to 1040-nm range. Theoretical predictions of the scaling theory on the number and intensity wavelength dependences of hot spots in the near-infrared are confirmed. Statistical properties of the intensity distribution, spectral behavior, and spatial localization of the hot spots are reported.
The growth and optical properties of semicontinuous silver films on insulator substrates were studied experimentally and theoretically. In the experimental studies, films were synthesized by the pulsed-laser deposition technique, characterized by electron microscopy and in situ optical and dc electrical resistance measurements and studied using near-field optical microscopy. The percolation threshold of the films was found to be at ∼65% metal filling fraction, higher than the 50%–60% range of values predicted for two-dimensional (2D) bond or site percolation films and suggesting the importance of grain coalescence and 3D growth in our system. Local optical properties measured by near-field optical microscopy were compared with theoretical results obtained using the block-elimination method, with good agreement. Local-field distributions were found to depend strongly on the metal concentration and wavelength of illumination. The degree of localization was found to increase at metal concentrations both above and below the percolation threshold. At a very high metal coverage, very strong local fields were observed in submicron voids of a metal dielectric film. These fields are likely due to localized surface plasmon polaritons.
Extracting characteristic dimensions from mounded surfaces such as grain size or intergrain lengths is usually made by statistical analysis. Different statistical functions are used in the literature to extract characteristic lengths. The main issue is that depending of the choice of the statistical function the results can be very different. In this paper, we demonstrate using a series of model mounded surfaces for which characteristic dimensions are known, that a method (namely Interfacial Differential Function) is the most effective method to determine the different characteristic lengths. The influence on the statistical treatment of the variation of the different characteristic lengths is then studies and confirms the ability of the IDF analysis. The IDF method was used to analyze the evolution of ultrathin gold film morphology as function of deposition temperature. This approach allows us to demonstrate that the roughness increase with deposition temperature is mainly due to a grain height increase and not to a grain coarsening phenomena as it was claimed before.
A systematic AFM study of gold films deposited on mica substrates under a range of selected conditions reveals the evolution of the surface morphology of the films as a function of growth temperature, film thickness, and deposition rate. Scaling analysis shows that the growth behaviour of the surfaces of these films can be considered as self-affine fractals. The observed topography follows island-type (3D) growth processes in the majority of cases with a roughness exponent, , in the range 0.24 - 0.75, or quasi-2D island growth in the remainder with or 1.45. In the latter cases, however, the two gold films grown under the particular conditions used reflect typical growth models expressing a high roughness exponent, well consistent with theoretical prediction.
This paper presents an in-depth study of Surface Enhanced Raman Scattering (SERS) enhancement factors (EFs) and cross-sections, including several issues often overlooked. In particular, various possible rigorous definitions of the SERS EFs are introduced and discussed in the context of SERS applications, such as analytical chemistry and single molecule SERS. These definitions highlight the importance of a careful characterization of the non-SERS cross-sections of the probes under consideration. This aspect is illustrated by experimental results for the non-SERS cross-sections of representative SERS probes along with average SERS EFs for the same probes. In addition, the accurate experimental determination of single molecule enhancement factors is tackled with two recently developed techniques, namely: bi-analyte SERS (BiASERS) and temperature-dependent SERS vibrational pumping. We demonstrate that SERS EFs as low as 10 7 , as opposed to the figure of 10 14 often claimed in the literature, are sufficient for the observation of single molecule SERS signals, with maximum single molecule EFs typically on the order of ∼10 10 .
Sputtered gold films in a pure form or as nanocomposites in silica or silicon nitride were screened for surface-enhanced Raman scattering (SERS) activity using Rhodamine 6G as a probe. The films were prepared by sputtering pure gold or solidified Au–Si alloys in plasmas generated in a dc glow discharge apparatus. The plasmas were produced with argon, nitrogen, or argon–oxygen as the sputtering gas to directly deposit gold films or in the latter case a gold oxide intermediate. The alloys produce nanocomposite films in a silicon nitride or silica matrix depending on the plasma gas. SERS activity was detected in some of the films thus leading to a search for the critical parameters that controlled this phenomenon. The films were characterized by profilometry, x-ray diffraction, and atomic force microscopy. SERS activity was found to be correlated to crystallite size in the 10–25 nm range and to roughness larger than 15 nm, and it was independent of film thickness. Sputtered gold films, particularly those containing the gold as a nanocomposite in silica are attractive media for SERS because of excellent adherence, ruggedness, and simplicity in preparation. © 1997 American Vacuum Society.
Raman spectroscopy has been employed for the first time to study the role of adsorption at electrodes. It has been possible to distinguish two types of pyridine adsorption at a silver electrode. The variation in intensity and frequency of some of the bands with potential in the region of the point of zero charge has given further evidence as to the structure of the electrical double layer; it is shown that the interaction of adsorbed pyridine and water must be taken into account.
The charged cluster model states that chemical vapor deposition (CVD) begins with gas-phase nucleation of charged clusters followed by cluster deposition on a substrate surface to form a thin film. Gold deposition by thermal evaporation was studied in a two-chambered CVD system in order to determine if this mechanism applies to the present case. The presence of nanometer-sized gold clusters was confirmed by transmission electron microscopy (TEM). The charge on the primary clusters was found to be positive. Smaller clusters were found to be amorphous and combined with clusters already deposited on a substrate surface to form larger amorphous clusters on the surface. Larger clusters were also observed by TEM to form after 120 and 300 s. These clusters exhibited a lattice structure and may have grown by attachment of small clusters in the gas phase.
By exploiting the extremely large effective cross sections ( 10-17-10-16 cm2/molecule) available from surface-enhanced Raman scattering (SERS), we achieved the first observation of single molecule Raman scattering. Measured spectra of a single crystal violet molecule in aqueous colloidal silver solution using one second collection time and about 2×105 W/cm2 nonresonant near-infrared excitation show a clear ``fingerprint'' of its Raman features between 700 and 1700 cm-1. Spectra observed in a time sequence for an average of 0.6 dye molecule in the probed volume exhibited the expected Poisson distribution for actually measuring 0, 1, 2, or 3 molecules.
In this work we have verified the remarkable sensitivity of Raman spectroscopy for the study of adsorbed pyridine on a silver surface, and extended its applicability to other nitrogen heterocycles and amines. New bands in the scattering spectrum of adsorbed pyridine have been characterized, which were not previously reported, as well as the Raman intensity response of all the surface pyridine bands as a function of electrode potential. As a result of these experiments, we have proposed a model of the adsorbed species for pyridine in which the adsorption is anion induced, leading to an axial end-on attachment to the electrode surface. The ability to obtain resonance Raman spectra with good signal-to-noise with laser powers less than 1.0 mW, reported here for the first time, opens up possibilities of surface Raman studies with relatively inexpensive laser systems. As laser power requirements are relaxed, reliability is improved, and greater tuning ranges can be achieved for wavelength dependent studies. We previously demonstrated the potential of resonance Raman spectroscopy for monitoring solution kinetic behavior [2], and now have shown that NR as well as RR spectroscopy has sufficient sensitivity to extend the studies of kinetic processes to include those occurring at electrode surfaces.
We report on the resolution limits of Electron Beam Lithography (EBL) in the conventional polymethylmethacrylate (PMMA) organic resist. We show that resolution can be pushed below 10 nm for isolated features and how dense arrays of periodic structures can be fabricated at a pitch of 30 nm, leading to a density close to 700 Gbit/in2. We show that intrinsic resolution of the writing in the resist is as small as 3 to 5 nm at high incident electron energy, and that practical resolution is limited by the development of the resist after exposure and by pattern transfer. We present the results of our optimized process for reproducible fabrication of sub-10 nm lines by lift-off and 30-nm pitch pillar arrays by lift-off and reactive ion etching (RIE). We also present some applications of these nanostructures for the fabrication of very high density molds for nano-imprint lithography (NIL) and for the fabrication of Multiple Tunnel Junction devices that can be used for single electron device applications or for the connection of small molecules.
Surface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the 'real' enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique.
Ag-capped Au nanopillar arrays on a resin supporter (see left upper figure), with a typical adjacent pillar tip gap of 10 nm, show obviously higher surface-enhanced Raman scattering (SERS) sensitivity (right column in red) than that of the bare Au nanopillar array while using 10 nM R6G as probe molecules. The large-area Ag-capped Au nanopillar array has potential in trace detection of special chemicals.
The surface structures, adsorption conditions, and thermal desorption behaviors of benzenethiol (BT) and benzenemethanethiol (BMT) self-assembled monolayers (SAMs) on Au(111) were examined by means of scanning tunneling microscopy (STM), X-ray photoelectron microscopy (XPS), and thermal desorption spectroscopy to understand the effects of the alkyl spacer between the phenyl group and the sulfur atom. Although XPS spectral shapes in the S 2p region for both SAMs are similar, the surface structures and thermal desorption behaviors differ significantly. BT SAMs on Au(111) were composed of disordered phases, whereas BMT SAMs have well-ordered phases containing vacancy islands. The strong desorption peak for parent mass species (m/z=110,C(6)H(5)SH(+)) was observed in BT SAMs at about 500K, whereas no desorption peak (m/z=124,C(6)H(5)CH(2)SH(+)) was observed from BMT SAMs. Interestingly, the dominant TD peak for the benzyl fragments (m/z=91,C(6)H(5)CH(2)(+)) formed via C-S bond cleavage was observed in BMT SAMs at around 400K. From this study, we clearly revealed that the small modification in chemical structure by inserting a methylene spacer between the phenyl group and the sulfur atom affects 2D SAM structures, adsorption conditions, and thermal desorption behaviors and stability. The results obtained here will be very useful in designing and fabricating aromatic thiol SAMs for further applications.
The development of quantitative, highly sensitive surface-enhanced Raman spectroscopy (SERS) substrates requires control over size, shape, and position of metal nanoparticles. Despite the fact that SERS has gained the reputation as an information-rich spectroscopy for detection of many classes of analytes, in some isolated instances down to the single molecule detection limit, its future development depends critically on techniques for nanofabrication. Herein, an unconventional nanofabrication approach is used to produce efficient SERS substrates. Metallic nanopatterns of silver disks are transferred from a stamp onto poly(dimethysiloxane) (PDMS) to create nanocomposite substrates with regular periodic morphologies. The stamp with periodic arrays of square, triangular, and elliptical pillars is created via electron beam lithography (EBL) of ma-N 2403 resist. A modified cyclodextrin is thermally evaporated onto the stamp to overcome the adhesive nature of the EBL resist and to function as a releasing layer. Subsequently, Ag is physically vapor deposited onto the stamp at a controlled rate and thickness and used directly for nanotransfer printing (nTP). Stamps, substrates, and the efficiency of the nTP process were explored by scanning electron microscopy. Transferred Ag nanodisk-PDMS substrates are studied by SERS using Rhodamine 6G as the probe analyte. There are observed optimal conditions involving both Ag and cyclodextrin thickness. The SERS response of metallic nanodisks of various shapes and sizes on the original stamp is compared to the corresponding nTP created substrates with similar trends observed. Limits of detection for crystal violet and Mitoxantrone are approximately 10(-8) and 10(-9) M, respectively. As an innovative feature of this approach, we demonstrate that physical manipulation of the PDMS post-nTP can be used to alter morphology, e.g., to change internanodisk spacing. Additionally, stamps are shown to be reusable after the nTP process, adding the potential to scale-up regular morphology substrates by a stamp-and-repeat methodology.
Metals thermally evaporated onto warm insulating substrates evolve to the thin-film state via the morphological sequence: compact islands, elongated islands, percolation, hole filling, and finally the thin-film state. The coverage at which the metal percolates ({ital p}{sub {ital c}}) is often considerably higher than that predicted by percolation models, such as inverse swiss cheese or lattice percolation. Using a simple continuum model, we show that high-{ital p}{sub {ital c}}'s arise naturally in thin films that exhibit a crossover from full coalescence of islands at early stages of growth to partial coalescence at later stages. In this interrupted-coalescence model, full coalescence of islands occurs up to a critical island radius {ital R}{sub {ital c}}, after which islands overlap, but do not fully coalesce. We present the morphology of films and the critical area coverages generated by this model.