Plasmonic Enhancement of Vibrational Excitations in the Infrared

Neubrech, Frank; Pucci, Annemarie

doi:10.1109/jstqe.2012.2227302

Cited by 55 publications

(85 citation statements)

References 78 publications

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“…13,31 The origin of such signal enhancement effects stems from the excitation of surface plasmon resonances in the nanoparticles which allows for significantly higher electric field strengths at the surface compared to the incident beams. 34 Regarding the experiments presented here, similar contributions can be expected due to the presence of rough surfaces which are generally obtained from sputter-coating sample preparation. 35 The details of the particular surface enhancement encountered in 2D ATR IR spectroscopy are currently under investigation and will be published independently.…”

Section: Discussionsupporting

confidence: 76%

Surface-enhanced, multi-dimensional attenuated total reflectance spectroscopy

2015

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Ultrafast two-dimensional infrared spectroscopy (2D IR) spectroscopy is performed in attenuated total reflectance (ATR) geometry with the Kretschmann configuration in order to measure femtosecond to picosecond dynamics of selfassembled monolayers on gold-coated solid-liquid interfaces. In the monolayers low-absorbing (<200 M-1 cm-1) nitrile functional groups are used as local vibrational probes to monitor vibrational relaxation and spectral diffusion in dependence of different environments of the nitrile group. By comparing spectral diffusion dynamics of the vibrational probe in bulk solution and in the monolayer we find that the dynamics are slowed down by more than a factor of 20 upon immobilization of the sample. Moreover, spectral diffusion dynamics are affected by the local environment within the monolayers as evidenced by 2D ATR IR experiments on mixed monolayers with different aliphatic and aromatic coadsorbates. The results are interpreted in terms of absent excitation energy-transfer as well as solvation dynamics around the nitrile vibrational probe. Our results demonstrate that 2D ATR IR spectroscopy offers the possibility to obtain ultrafast dynamics from sub-monolayer coverages of even low-absorbing vibrational probes such as nitrile functional groups. ABSTRACTUltrafast two-dimensional infrared spectroscopy (2D IR) spectroscopy is performed in attenuated total reflectance (ATR) geometry with the Kretschmann configuration in order to measure femtosecond to picosecond dynamics of selfassembled monolayers on gold-coated solid-liquid interfaces. In the monolayers low-absorbing (<200 M -1 cm -1 ) nitrile functional groups are used as local vibrational probes to monitor vibrational relaxation and spectral diffusion in dependence of different environments of the nitrile group. By comparing spectral diffusion dynamics of the vibrational probe in bulk solution and in the monolayer we find that the dynamics are slowed down by more than a factor of 20 upon immobilization of the sample. Moreover, spectral diffusion dynamics are affected by the local environment within the monolayers as evidenced by 2D ATR IR experiments on mixed monolayers with different aliphatic and aromatic coadsorbates. The results are interpreted in terms of absent excitation energy-transfer as well as solvation dynamics around the nitrile vibrational probe. Our results demonstrate that 2D ATR IR spectroscopy offers the possibility to obtain ultrafast dynamics from sub-monolayer coverages of even low-absorbing vibrational probes such as nitrile functional groups.

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Section: Discussionsupporting

confidence: 76%

Surface-enhanced, multi-dimensional attenuated total reflectance spectroscopy

2015

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“…In the absence of the antennas or when the light is polarized perpendicular to the antenna axis, the enhancement vanishes. 6,17 While many facts indicate the particular importance of the plasmonic scattering in SEIRA, it is not yet clear how plasmonic scattering and plasmonic absorption mutually contribute to SEIRA that relies on a signal Figure 1. (a) Quasi-static fit to the experimental transmittance (top row) of single nanowires at normal incidence of IR light and to FDTD simulation of such spectra (bottom row) for the same geometries (length l, height h, and width w, given in the panels).…”

Section: ■ Introductionmentioning

confidence: 99%

“…4 Less than 10 years ago, a stronger SEIRA (by almost 6 orders of magnitude) from molecules on micrometer long metal nanowires with their plasmonic resonances matching the vibrational frequencies was detected and identified as a Fano effect with the plasmonic excitation spectrum as the background. 5,6 The Fano-type coupling between this background and the vibrationally induced dipole is governed by the nearfield strength (apart from the vibrational dipole moment) as it becomes obvious upon detuning of the two excitation spectra …”

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confidence: 99%

Importance of Plasmonic Scattering for an Optimal Enhancement of Vibrational Absorption in SEIRA with Linear Metallic Antennas

et al. 2015

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Surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman scattering (SERS) represent very effective techniques to detect molecular vibrational fingerprints. These techniques can be improved thanks to the use of plasmonic antennas that produce strong resonant near-fields in their vicinity, enhancing the signal of vibrational samples. Here we study the role of plasmonic absorption and scattering of the hosting antennas in the resulting SEIRA signal. Using numerical simulations of the antenna−sample infrared response, we show that the optimal SEIRA signal measured in transmittance (as extinction) is achieved when the spectral maxima of absorption and scattering of the antennas are of similar magnitude. Paradoxically, when the optimal condition for SEIRA is fulfilled, the decomposition of the signal into the contribution from scattering and from absorption show that the vibrational fingerprint is exclusively a result of the scattering, with no contribution from absorption. Using a simple analytical model for the description of the fundamental resonance of linear nanoantennas made of a Drude-type metal, we provide guidelines for controlling the plasmonic light scattering and light absorption properties, thus showing how the optimal condition for SEIRA can be achieved in practical situations. ■ INTRODUCTIONInfrared (IR) vibrational spectroscopy is a powerful tool for the identification of the chemical composition and the molecular geometry via the vibrational fingerprints. However, one big problem hampers the successful application of IR spectroscopy, especially in the case where small amounts of molecules are present. This problem consists in the extremely small IR absorption cross section of molecules which is much smaller than the squared wavelength of the probing IR radiation at the vibrational frequency of the molecule (see Supporting Information). This problem can be overcome by placing the molecule into strongly enhanced electromagnetic near-fields, for example, such of a resonant plasmonic antenna. In this article we will demonstrate that the resonant plasmonic scattering of these hosting antennas is especially relevant for the measurement of enhanced IR vibrational absorption signals.The first findings on enhanced signals of molecules on metal nanoparticles, ca. 50 years ago, were named "anomalous transmission".1 This kind of surface-enhanced IR absorption (SEIRA) of adsorbates on metal−nanoparticle aggregates can reach 3 orders of magnitude and can be modeled by effective media theories (EMT) if the particles are small enough. Usually, the metal nanoparticles in such SEIRA studies have diameters of only a few nanometers, and therefore their light scattering (proportional to the fourth power of the ratio of the particle diameter to the wavelength) almost vanishes in the IR. On the other hand, the optical absorption of these structures even in the IR is strong because the mutual interaction of the nanoparticles broadens and red-shifts their resonances.3 The observed vibrational line-sh...

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“…While early studies have relied on metal island films, prepared either by physical vapor deposition or chemical means [15][16][17][18], recent work has shown that nanofabricated plasmonic nanoantennas are an extremely promising approach, with a number of important advantages [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. These include improved repeatability [34,35,37], deterministic and well-defined sensing regions [19,20,24,32,[34][35][36] and dramatically higher absorption signal enhancement factors [19][20][21][22][23][25][26][27][32][33][34][35][36]38]. The increase in enhancement is most notable as it is a result of an important fundamental difference between the two approaches.…”

Section: Introductionmentioning

confidence: 99%

“…Specifically, due to particle sizes and geometrical fluctuations in metal island films, generally the long wavelength tail of a stochastic collection of plasmon resonances is responsible for field enhancement. In contrast, nanoantennas can be engineered to support a single, welldefined resonance directly at the frequency of interest, leading to a resonant coupling between the absorption line and the plasmonic mode [22,24,25,[27][28][29][30][31][32][33][34][35][36]39]. In this review we discuss the basic principles and key focus areas essential to moving this resonant SEIRA approach forward as a promising technique applied in various fields including biotechnology, pharmacology, biomedical research and biology to yield new insights.…”

Section: Introductionmentioning

confidence: 99%

Engineering mid-infrared nanoantennas for surface enhanced infrared absorption spectroscopy

2015

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Mid-infrared absorption spectroscopy is a powerful tool for optically probing the molecular structure of biological samples. However, using conventional approaches, IR measurements on small quantities of molecules are extremely challenging due to sensitivity limitations. The strong IR absorption of liquid water presents additional obstacles to performing measurements in biomolecules' native, aqueous environments. In this review we discuss the application of engineered plasmonic nanoantennas to overcoming these challenges. We provide overviews and highlight the key concepts of our recent work in designing infrared antennas and applying them to enhance the absorption of minute quantities of biomolecules as well as new fabrication methods for their high throughput and low-cost manufacturing.

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Plasmonic Enhancement of Vibrational Excitations in the Infrared

Cited by 55 publications

References 78 publications

Surface-enhanced, multi-dimensional attenuated total reflectance spectroscopy

Surface-enhanced, multi-dimensional attenuated total reflectance spectroscopy

Importance of Plasmonic Scattering for an Optimal Enhancement of Vibrational Absorption in SEIRA with Linear Metallic Antennas

Engineering mid-infrared nanoantennas for surface enhanced infrared absorption spectroscopy

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