Interparticle coupling effects in nanofabricated substrates for surface-enhanced Raman scattering

Gunnarsson, Linda; Bjerneld, Erik J.; Xu, Hongxing; Petronis, Šarūnas; Kasemo, Bengt Herbert; Käll, Mikael

doi:10.1063/1.1344225

Cited by 426 publications

(418 citation statements)

References 9 publications

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“…In contrast to earlier experimental and theoretical work that suggested that the SERS enhancement is largest for junctions with the smallest gap (10,(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25), here we find a non-zero optimum gap size for a specified excitation wavelength that results from the red-shifting of the dipole plasmon wavelength as gap size is decreased. Second, we show that there is also an optimum particle size that reflects the ability of light to penetrate into and be confined by the gap structure.…”

Section: Resultscontrasting

confidence: 53%

“…Two mechanisms are often mentioned in the literature, (6) the electromagnetic mechanism and the chemical mechanism. The latter involves charge transfer excitation (7, 9) between analyte molecules and the metal particles, whereas the former is dominated by plasmon excitation leading to hot spots around nano-sized metal particles (10,(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). A challenge in characterizing the electromagnetic mechanism is the difficulty associated with making nanoparticle structures with controllable interparticle gaps and doing so in a way that does not significantly change chemical composition.…”

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confidence: 99%

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Designing, fabricating, and imaging Raman hot spots

Qin

Zou

Xue

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

431

474

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We have developed a probe of the electromagnetic mechanism of surface-enhanced Raman scattering via Au nanodisk arrays generated by using on-wire lithography. In this approach, disk thickness and interparticle gap are precisely controlled from 5 nm to many micrometers. Confocal Raman microscopy demonstrates that disk thickness and gap play a crucial role in determining surfaceenhanced Raman scattering intensities. Theoretical calculations also demonstrate that these results are consistent with the electromagnetic mechanism, including the surprising result that the largest enhancement does not occur for the smallest gaps.electromagnetic mechanism ͉ nanofabrication ͉ surface-enhanced Raman scattering ͉ templated synthesis ͉ discrete dipole approximation O n-wire lithography (OWL) allows one to fabricate unique one-dimensional structures that cannot be prepared via any other lithographic method (1-3). In particular, it allows one to make nanodisk arrays coated on one side with a thin silica sheath where the disk composition, thickness, and separation along the long axis can be controlled with nanometer precision. This ability enables the exploration of a variety of chemical and physical phenomena, including plasmon coupling and electromagnetic field enhancement in a very unique manner. Electromagnetic field enhancement is, in part, the basis behind surface-enhanced Raman scattering (SERS), a spectroscopic phenomenon discovered over 30 years ago but still perplexing to the scientific community both in terms of its potential and scientific origins (4-27). Two mechanisms are often mentioned in the literature, (6) the electromagnetic mechanism and the chemical mechanism. The latter involves charge transfer excitation (7, 9) between analyte molecules and the metal particles, whereas the former is dominated by plasmon excitation leading to hot spots around nano-sized metal particles (10,(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). A challenge in characterizing the electromagnetic mechanism is the difficulty associated with making nanoparticle structures with controllable interparticle gaps and doing so in a way that does not significantly change chemical composition. Because of this problem, it would be particularly useful to be able to make a series of nearly identical nanostructures with controllable gap size that can be probed simultaneously under one set of conditions in a SERS experiment. By designing the experiment in this manner, differences in chemical enhancement are minimized, and one can focus on the relationship between nanostructure and electromagnetic enhancement. Here, we show how OWL can be used to systematically prepare rows of Au disks with precisely controlled thicknesses and gaps, enabling a combinatorial format for identifying structures that provide maximum SERS enhancement.Nanodisk arrays fabricated by OWL are particularly useful for preparing SERS-active nanostructures for the following reasons. (i) Multiple features can be synthesized within a single nanowire, and, once functionalized wi...

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Section: Resultscontrasting

confidence: 53%

mentioning

confidence: 99%

Designing, fabricating, and imaging Raman hot spots

Qin

Zou

Xue

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

431

474

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“…As reported, this structure is more approachable than the theoretical model that an analyte molecule positioned between two noble metal NPs can be enhanced greatly. 52 So one of the most interesting possibilities about the method of fabricating sandwich structures on glass is to achieve a kind of electromagnetic coupling effect between the two layers of metal NPs through controlling the assembly process, and this effect is known to give a dominant contribution to the SERS efficiency. 49 Therefore, large EF values are estimated to exist in such sandwich structure.…”

Section: Resultsmentioning

confidence: 99%

Surface enhanced Raman scattering of p-aminothiophenol self-assembled monolayers in sandwich structure fabricated on glass

Wang

Chen

Dong

et al. 2006

The Journal of Chemical Physics

107

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A sandwich structure consisting of Ag nanoparticles (NPs), p-aminothiophenol (p-ATP) self-assembled monolayers (SAMs), and Ag NPs was fabricated on glass and characterized by surface enhanced Raman scattering (SERS). The SERS spectrum of a p-ATP SAM in such sandwich structure shows that the electromagnetic enhancement is greater than that on Ag NPs assembled on glass. The obtained enhancement factors (EF) on solely one sandwich structure were as large as 6.0±0.62×104 and 1.2±0.62×107 for the 7a and 3b(b2) vibration modes, respectively. The large enhancement effect of p-ATP SAMs is likely a result of plasmon coupling between the two layers of Ag NP (localized surface plasmon) resonance, creating a large localized electromagnetic field at their interface, where p-ATP resides. Moreover, the fact that large EF values (∼1.9±0.7×104 and 9.4±0.7×106 for the 7a- and b2-type vibration modes, respectively) were also obtained on a single sandwich structure of Au NPs∕p-ATPSAMs∕Ag NPs in the visible demonstrates that the electromagnetic coupling does not exist only between Ag NPs but also between Au and Ag NPs. The lower EF values on Au-to-Ag NPs compared to those on Ag-to-Ag NPs demonstrate that the Au-to-Ag coupling must be less effective than the Ag-to-Ag coupling for the induction of SERS in the visible.

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“…Recently, electronbeam lithography substrates have been used to explore the magnitude of the enhancement factor while the interparticle spacing is varied; enhancement factors as large as 10 8 have been achieved (32,33). Immobilized colloidal nanoparticles have been used as a SERS substrate for the analysis of the eluent in the CE separation of small molecules, such as amino acids (34 ).…”

Section: Sers Substratesmentioning

confidence: 99%

Surface-Enhanced Raman Spectroscopy

Haynes¹,

McFarland²,

Duyne³

2005

Anal. Chem.

1,072

855

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mekal theoretically predicted inelastic light scattering in 1923 (1). Raman and Krishnan first experimentally observed the phenomenon and reported in their 1928 Nature paper that the inelastic scattering effect was characterized by "its feebleness in comparison with the ordinary scattering" (2). This "feeble" phenomenon is now known as Raman scattering. The change in wavelength that is observed when a photon undergoes Raman scattering is attributed to the excitation (or relaxation) of vibrational modes of a molecule. Because different functional groups have different characteristic vibrational energies, every molecule has a unique Raman spectrum. In accordance with the Raman selection rule, the molecular polarizability changes as the molecular vibrations displace the constituent atoms from their equilibrium positions. The intensity of Raman scattering is proportional to the magnitude of the change in molecular polarizability. Thus, aromatic molecules exhibit more intense Raman scattering than aliphatic molecules.Even so, Raman scattering cross sections are typically 14 orders of magnitude smaller than those of fluorescence; therefore, the Raman signal is still several orders of magnitude weaker than the fluorescence emission in most cases. Because of the inherently small intensity of the Raman signal, the sensitivity limits of available detectors, and the intensity of the excitation sources, the applicability of Raman scattering was restricted for many years. However, its utility as an analytical technique improved with the advent of the laser and the evolution of photon detection technology.In 1977, Jeanmaire and Van Duyne demonstrated that the magnitude of the Raman scattering signal can be greatly enhanced when the scatterer is placed on or near a roughened noble-metal substrate (3). Strong electromagnetic fields are generated when the localized surface plasmon resonance (LSPR) of nanoscale roughness features on a silver, gold, or copper substrate is excited by visible light. When the Raman scatterer is subjected to these intensified electromagnetic fields, the magnitude of the induced dipole increases, and accordingly, the intensity of the inelastic scattering increases. This enhanced scattering process is known as surface-enhanced Raman (SER) scattering-a term that emphasizes the key role of the noblemetal substrate in this phenomenon.

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Interparticle coupling effects in nanofabricated substrates for surface-enhanced Raman scattering

Cited by 426 publications

References 9 publications

Designing, fabricating, and imaging Raman hot spots

Designing, fabricating, and imaging Raman hot spots

Surface enhanced Raman scattering of p-aminothiophenol self-assembled monolayers in sandwich structure fabricated on glass

Surface-Enhanced Raman Spectroscopy

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