Principles Of Surface Enhanced Raman Spectroscopy And Related Plasmonic Effects Pdf
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- Principles of Surface-Enhanced Raman Spectroscopy: And Related Plasmonic Effects
- Fundamental understanding and applications of plasmon-enhanced Raman spectroscopy
- Principles of Surface-Enhanced Raman Spectroscopy
- Surface-Enhanced Raman Scattering: Introduction and Applications
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Principles of Surface-Enhanced Raman Spectroscopy: And Related Plasmonic Effects
Tip enhanced Raman scattering TERS is an emerging technique that uses a metalized scanning probe microscope tip to spatially localize electric fields that enhances Raman scattering enabling chemical imaging on nanometer dimensions. Arising from the same principles as surface enhanced Raman scattering SERS , TERS offers unique advantages associated with controling the size, shape, and location of the enhancing nanostructure. The relationship between plasmon resonances and Raman enhancements is emphasized as the key to obtaining optimal TERS results.
Applications of TERS, including chemical analysis of carbon nanotubes, organic molecules, inorganic crystals, nucleic acids, proteins, cells and organisms, are used to illustrate the information that can be gained. Under ideal conditions TERS is capable of single molecule sensitivity and sub-nanometer spatial resolution. The ability to control plasmonic enhancements for chemical analysis suggests new experiments and opportunities to understand molecular composition and interactions on the nanoscale.
The near-field interaction between light and metallic, or metalized, scanning probe microscope tips has transformed Raman scattering into a method capable of providing chemical specificity, single molecule sensitivity, and nanometer spatial resolution. Since the initial demonstration of tip-enhanced Raman scattering TERS more than a decade ago [1—3], significant progress has been made in our understanding of the mechanisms that give rise to these enhancements and also the utilization of these enhancements in a variety of applications.
Some of the most impressive results include single nanometer resolution and single molecule detection [4—7]. A variety of different experimental configurations have been developed to address different chemical questions.
It is the intent of this review to assess current understanding of the mechanisms that give rise to TERS, to show how TERS has been applied to chemical analysis, and to address the future potential for Raman on the nanoscale. The initial demonstrations of TERS involved bringing an etched gold wire or silver nanoparticle vapor deposited onto the apex of an atomic force microscope AFM tip into the focus of a Raman spectrometer and collecting the enhanced Raman scattering [1—3].
Models have shown that an etched metal tip and metal nanoparticle behave qualitatively similar, with respect to the electric field induced by a localized surface plasmon resonance . In either case, the local electric field generated in TERS tip enhances the Raman scattering from molecules in close proximity, generally a few nanometers, or on the nanoscale. It was recognized that enhancements in SERS are correlated to plasmon resonances .
The advent of controlled nanostructures, with well-defined plasmon resonances, transformed SERS into an ultrasensitive detection method [10—17], which includes TERS. Mie theory calculations for a small spherical silver particle indicate the electromagnetic enhancement achieved when the plasmon resonance is excited is on the order of 10 4 —10 5 .
This enhanced electric field decays quite rapidly, localizing signals to the dimensions of the particle . Pointed metal probes have also been investigated for other near-field microscopies . Illumination of pointed probes has been shown to concentrate the electromagnetic radiation at the apex of the tip . The local field at the apex provides a near-field light source, which can induce scattering on the dimensions of the tip. Inspired by the invention of the scanning tunneling microscope STM , Wessel postulated that the Raman scattering from an excited nanoparticle on the apex of an STM tip could be used for nanoscale Raman microscopy .
Kawata and Inouye further suggested the enhanced fields in apertureless near-field microscopy correlated with surface enhanced spectroscopies . Ultimately it was shown that moving the nanoparticle on the end of a scanning probe microscope tip or a pointed metal probe, enables mapping of chemical composition with spatial resolution below the optical diffraction limit [23—25].
A thorough review of SERS is beyond the scope of this article and excellent reviews on the topic are available [11, 26, 27]; however, it is useful to discuss some key points to facilitate the discussion of TERS.
The collective excitation of conduction band electrons in a nanostructure, an LSPR, results in a strong local electric field.
Since Raman scattering is proportional to the electric field, molecules that experience this increased local electric field generate enhanced scattering. The scattering intensity is wavelength dependent, and the plasmon resonance frequency varies with different metals. Figure 1 shows the resulting field around a gold nanoparticle and the relationship between polarization and the enhanced electric field.
The plasmon mode is excited along the incident laser polarization and directs the orientation of the electric field around the nanoparticle. The Mie Scattering calculation of the electric field around a 40 nm gold nanoparticle excited by a nm plane wave shows the enhancement along the polarization vector of the excitation field. Empirical measurements have shown the SERS intensity decays rapidly as you move some distance d away from the nanoparticle with radius r :.
The distance dependence of the enhanced field provides a first order estimate for the spatial resolution in TERS imaging. The relationship between curvature and signal decay suggests that using sharp points and small particles as TERS tips will improve imaging resolution.
In addition to amplifying the excitation field, the LSPR acts as an antenna and will re-radiate the Raman scattering from molecules in the near field. The combination of these two effects is commonly referred to as the E 4 approximation [31, 32], where the enhancement factor EF is governed by these two separate effects as shown in equation In equation 2, the change in the electric field that excites the molecules E exc and the emitted Raman photon E emm are normalized to the incident laser E 0.
Because the wavelength difference between the excitation and emission is generally small, it is often ignored; however, enhancement bandwidth can vary if the plasmon resonance is not broad relative to the difference in these frequencies. The increases in electric fields are the calculated enhancement relative to spontaneous Raman, which arises from incident radiation E 0.
As illustrated in Figure 1 , the electromagnetic enhancement attendant to a small spherical particle is small and dependent upon illumination at its plasmon resonance frequency. For an isolated particle with proper excitation, enhancement factors are a modest 10 5 . To put this enhancement factor into context, an enhancement of 10 8 is needed to detect the Raman scattering from a single rhodamine 6G molecule .
The transformation of TERS into a single molecule method requires additional enhancement. The signal from hotspots is reported to dominate the observed Raman spectrum .
An electromagnetic field of 10 8 is commonly obtained in a 1 nm gap between two silver nanoparticles . Work by Pettinger and colleagues, has shown that as the TERS tip approaches a metallic surface, an image dipole is induced in the surface that acts in the manner of a second nanoparticle [38, 39].
The gap resonance frequency is a function of tip-substrate separation, the size of TERS tip, and metal s used. Figure 2 shows the parallels between the gap mode configuration and the classic nanoparticle dimer. The calculation in Figure 2 was performed for gold nanoparticles with nm excitation, a common experimental condition. The scattered electric field is an order of magnitude larger in the gap junction than around an isolated nanoparticle.
With nm nanoparticles, the calculated scattered electric field is even larger, owing to improved overlap between the excitation laser and emitted Raman photons with the plasmon resonance. Proper configuration of the gap provides an enhancement that is sufficiently strong to detect the Raman scattering from a single molecule . In the absence of a gap-mode, most TERS measurements require acquisitions on the order of a few seconds to generate appreciable signals from collections of molecules [47, 48].
A The conceptual illustration of the gap mode described by Pettinger and workers is compared with B the Mie scattering calculation of two 40 nm gold spheres separated by 2 nm and nm plane wave excitation. The calculation uses identical conditions to Figure 1 and shows the field confined within the gap increases by an order of magnitude compared with the isolated particle. Panel A is reproduced with permission from Ref. From Eq. Computational work by Schatz and coworkers has further shown the importance of dipole re-radiation E emm observed in Raman enhancement .
Work by LeRu and Etchegoin has shown that E 4 is only true for a molecule with its transition dipole parallel to the electric field that results from a polarization parallel to the aligned NPs, thereby maximizing coupling of the electric fields [50, 51]. The importance of overlap with the plasmon resonance has had interesting consequences for the Raman signals observed in SERS.
Early SERS studies in aggregated nanoparticles showed that enhancement correlated with overlap with plasmon modes . Van Duyne and colleagues further showed that maximum enhancement arose when exciting Raman with a laser frequency at higher energy than the plasmon mode, thereby increasing overlap with the emitted Raman scattering . Recently, Halas and Nordlander have investigated the interference between plasmons in aggregated systems and shown dramatic changes in the observed SERS spectrum .
The importance of the overlap between the electric fields and the Plasmon resonance has also impacted TERS results. A striking recent result, overlap of the gap-resonance with the Raman emission was shown to be critical in obtaining sub-nanometer TERS spatial resolution . The resolution demonstrated is better than would be predicted by Eq. A quick inspection of the electric field intensity calculated in Figure 2B suggests the highest enhancement is confined to a few nanometers.
Other work has shown that dramatic signal increases can be obtained in TERS from increased overlap with plasmon resonances. It was shown that varying the thickness of a metal film beneath a sharp metal TERS tip tuned the resonance associated with the gap and impacted the TERS response .
The red-shifted plasmon resonance from the interaction of a nanoparticle TERS tip and an isolated nanoparticle results in increased TERS emission when excited with a red laser . The interactions with the resulting plasmon resonances from interacting nanostructures appears to be an important factor for high sensitivity and high resolution TERS. Whereas the understanding of SERS effects benefitted from reproducible nanostructures, TERS offers the additional advantage of controlled manipulation of the nanostructures.
TERS imaging of chemical properties at the nanoscale derives from the ability to precisely position the metal nanostructure. This ability to control the position of the nanoparticle opens additional avenues of research unique to TERS. The controlled interaction between two nanoparticles is one example.
SERS experiments rely on searching to find nanoparticle aggregates that can be modeled to explain observed Raman enhancements and plasmonic effects. TERS experiments have shown the same effects in a reproducible manner. Olk and coworkers demonstrated increased Raman enhancements observed in dimer particles by moving a nanoparticle on the apex of an AFM tip over a nanoparticle on a substrate . They were able to match the observed Raman enhancements to the expected fields calculated in finite element models.
The plasmon resonance that acts to enhance Raman scattering has been calculated to depend on the size, shape, and position of nanoparticles in simple dimer assemblies  as well as complex aggregates [52, 57].
Different size and shape nanostructures can also be used for TERS tips. Given what is known about plasmon resonances in nanoparticles and the impact on Raman enhancements, tip construction clearly plays a role in TERS signals.
A numerical simulation comparing pointed probes, spherical nanoparticles, and triangular nanoparticles as TERS tips showed differences in the enhanced bandwidth associated with different tips .
An experimental comparison of the plasmon resonances, Raman enhancements and image resolution with different types of TERS tips has not been reported; nonetheless, the ability to control the nanostructure shape is a unique aspect of TERS.
The ability to use an STM tip for TERS has enabled other interesting experiments associated with tunneling currents through gap junctions. Zhang and coworkers noted a change in TERS signal intensity associated with the tunneling current in their experiments . It was reported that tunneling occurs in nanometer gaps associated with significant changes to plasmon resonances observed .
In this tunneling experiment, two Au nanoparticles on conductive AFM tips were manipulated with respect to each other while monitoring the plasmon resonances and tunneling current. In addition to plasmonic changes, applying a bias across a gap with simultaneous conductance monitoring shows changes in the TERS signal attributed to molecules in on and off states .
Given the connections between Raman enhancements and plasmon modes, the ability to control nanostructures enables TERS to provide unique insights into fundamentals associated with plasmon-enhanced spectroscopies. To take advantage of enhanced Raman signal from TERS, a number of different instrument configurations have been developed to address different experiments. Perhaps the most fundamental difference is the use of a scanning tunneling microscope STM or atomic force microscopy AFM to control the position of the tip.
The difference in feedback mechanism provides utility in different experiments. The tunneling current can regulate the height of an STM tip over the surface, which can be beneficial for tuning gap resonances.
Each approach has its advantages and both approaches have yielded impressive results. A second consideration is how the excitation laser couples with the metallic tip to promote enhancement of the sample. The polarization of light, as shown above, orients the electric fields around the TERS tip.
In general a polarization normal to the surface is required for optimal results . While focusing linear polarized light will enhance scattering from samples in close proximity to the tip , it has been shown that a radial polarized laser beam will generate a longitudinal mode at the focus of the laser beam increasing the interaction [20, 67, 68].
Fundamental understanding and applications of plasmon-enhanced Raman spectroscopy
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. Ru and P. Etchegoin Published Materials Science. Surface-Enhanced Raman Scattering SERS was discovered in the s and has since grown enormously in breadth, depth, and understanding. One of the major characteristics of SERS is its interdisciplinary nature: it lies at the boundary between physics, chemistry, colloid science, plasmonics, nanotechnology, and biology. By their very nature, it is impossible to find a textbook that will summarize the principles needed for SERS of these rather dissimilar and disconnected topics.
SERS was discovered in the s and has since grown enormously in breadth, depth, and understanding. One of the major characteristics of SERS is its interdisciplinary nature: it lies at the boundary between physics, chemistry, colloid science, plasmonics, nanotechnology, and biology. By their very nature, it is impossible to find a textbook that will summarize the principles needed for SERS of these rather dissimilar and disconnected topics. Although a basic understanding of these topics is necessary for research projects in SERS with all its many aspects and applications, they are seldom touched upon as a coherent unit during most undergraduate studies in physics or chemistry. This book intends to fill this existing gap in the literature. It provides an overview of the underlying principles of SERS, from the fundamental understanding of the effect to its potential applications.
Tip enhanced Raman scattering TERS is an emerging technique that uses a metalized scanning probe microscope tip to spatially localize electric fields that enhances Raman scattering enabling chemical imaging on nanometer dimensions. Arising from the same principles as surface enhanced Raman scattering SERS , TERS offers unique advantages associated with controling the size, shape, and location of the enhancing nanostructure. The relationship between plasmon resonances and Raman enhancements is emphasized as the key to obtaining optimal TERS results. Applications of TERS, including chemical analysis of carbon nanotubes, organic molecules, inorganic crystals, nucleic acids, proteins, cells and organisms, are used to illustrate the information that can be gained. Under ideal conditions TERS is capable of single molecule sensitivity and sub-nanometer spatial resolution. The ability to control plasmonic enhancements for chemical analysis suggests new experiments and opportunities to understand molecular composition and interactions on the nanoscale. The near-field interaction between light and metallic, or metalized, scanning probe microscope tips has transformed Raman scattering into a method capable of providing chemical specificity, single molecule sensitivity, and nanometer spatial resolution.
example, molecular Raman spectroscopy or the physics of plasmon resonances in metals. heard about surface-enhanced Raman spectroscopy (SERS) only superficially is, in fact, a shorthand for a family of effects associated with the.
Principles of Surface-Enhanced Raman Spectroscopy
Surface-Enhanced Raman Scattering: Introduction and Applications
Scattering of light by molecules can be elastic, Rayleigh scattering, or inelastic, Raman scattering. Hence, Rayleigh scattered light does not contain much information on the structure of molecular states. In inelastic scattering, the frequency of monochromatic light changes upon interaction with the vibrational states, or modes, of a molecule. With the advancement in the laser sources, better and compact spectrometers, detectors, and optics Raman spectroscopy have developed as a highly sensitive technique to probe structural details of a complex molecular structure. With the discovery of surface-enhanced Raman scattering SERS in by Martin Fleischmann, the interest of the research community in Raman spectroscopy as an analytical method has been revived. This chapter aims to familiarize the readers with the basics of Raman scattering phenomenon and SERS.
Molecular detection techniques are conventionally based on optical, electrochemical, electronic, or gravimetric methodologies. Unfortunately, the applicability of SERS is rather limited, which is mainly due to the lack of highly sensitive SERS platforms with good stability and reproducibility. In line with this, metal nanoparticles e. Although the utilization of metallic nanoparticles in SERS is simple and cost-effective, the poor controllability of the structures and limited formation of hot spots in the detection zone leads to discrepancy in the resulting SERS signals.
- Сьюзан пожала плечами, демонстрируя равнодушие. - Мы с ним какое-то время переписывались, - как бы невзначай сказал Хейл. - С Танкадо. Ты знала об. Сьюзан посмотрела на него, стараясь не показать свое изумление. - Неужели.
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У меня есть кое-что для .
Сьюзан, в свою очередь, удивил ответ шефа. - Но ведь у нас есть ТРАНСТЕКСТ, почему бы его не расшифровать? - Но, увидев выражение лица Стратмора, она поняла, что правила игры изменились. - О Боже, - проговорила Сьюзан, сообразив, в чем дело, - Цифровая крепость зашифровала самое. Стратмор невесело улыбнулся: - Наконец ты поняла. Формула Цифровой крепости зашифрована с помощью Цифровой крепости.
Он проявил редкую наблюдательность. - Но ведь вы ищете ключ к шифру, а не ювелирное изделие. - Конечно. Но я думаю, что одно с другим может быть связано самым непосредственным образом. Сьюзан отказывалась его понимать.
Колокола звонили где-то совсем рядом, очень громко. Беккер чувствовал жжение в боку, но кровотечение прекратилось. Он старался двигаться быстрее, знал, что где-то позади идет человек с пистолетом.