Nanooptics of Molecular-Shunted Plasmonic Nanojunctions

Benz, Felix; Tserkezis, Christos; Herrmann, Lars O.; Nijs, Bart de; Sanders, Alan; Sigle, Daniel O.; Pukenas, Laurynas; Evans, Stephen D.; Aizpurua, Javier; Baumberg, Jeremy J.

doi:10.1021/nl5041786

Cited by 162 publications

(217 citation statements)

References 43 publications

(70 reference statements)

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“…This finding is in stark contrast to recent work of Tan et al [Science 343, 1496[Science 343, (2014 [1,[3][4][5][6]. Molecular tunnel junctions enable tunneling over larger gap distances in the nanometer regime [7,8], and thus establish a novel platform for hybrid structures reconciling molecular electronics with plasmonics.Recent years have seen significant research efforts to understand the properties of gap plasmons, and have highlighted the importance of the tunneling strength as a trigger for the CTP appearance [9] and of the gap morphology which strongly influences the CTP spectral position [10]: for rounded gap terminations the bonding mode red-shifts with decreasing gap separation, until tunneling sets in, as evidenced by the appearance of a low-frequency CTP together with a broadening and blueshift of the bonding mode [1,3,6]. In contrast, for flat terminations the red-shift of the bonding mode saturates with decreasing gap distance, while at the same time the transversal cavity plasmon (TCP) modes shift to the red; here, the onset of tunneling has no significant impact on the bonding mode and no low-frequency CTP appears in the spectra.…”

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confidence: 60%

“…For sufficiently narrow gaps, electrons can tunnel directly from one nanoparticle to the other one, leading to the emergence of new charge transfer plasmons (CTPs) [1, [3][4][5][6]. Molecular tunnel junctions enable tunneling over larger gap distances in the nanometer regime [7,8], and thus establish a novel platform for hybrid structures reconciling molecular electronics with plasmonics.…”

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Gap plasmonics of silver nanocube dimers

et al. 2016

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We theoretically investigate gap plasmons for two silver nanocubes coupled through a molecular tunnel junction. In absence of tunneling, the red-shift of the bonding mode saturates with decreasing gap distance. Tunneling at small gap distances leads to a damping and slight blue-shift of the bonding mode, but no low-energy charge transfer plasmon mode appears in the spectra. This finding is in stark contrast to recent work of Tan et al. [Science 343, 1496[Science 343, (2014 [1,[3][4][5][6]. Molecular tunnel junctions enable tunneling over larger gap distances in the nanometer regime [7,8], and thus establish a novel platform for hybrid structures reconciling molecular electronics with plasmonics.Recent years have seen significant research efforts to understand the properties of gap plasmons, and have highlighted the importance of the tunneling strength as a trigger for the CTP appearance [9] and of the gap morphology which strongly influences the CTP spectral position [10]: for rounded gap terminations the bonding mode red-shifts with decreasing gap separation, until tunneling sets in, as evidenced by the appearance of a low-frequency CTP together with a broadening and blueshift of the bonding mode [1,3,6]. In contrast, for flat terminations the red-shift of the bonding mode saturates with decreasing gap distance, while at the same time the transversal cavity plasmon (TCP) modes shift to the red; here, the onset of tunneling has no significant impact on the bonding mode and no low-frequency CTP appears in the spectra.In this paper, we theoretically investigate the plasmonic properties of two coupled silver nanocubes, similarly to the electron energy loss spectroscopy (EELS) experiments of Tan et al. [7] for two nanocubes coupled through a molecular tunnel junction. We compute EEL and extinction spectra using the MNPBEM toolbox [11][12][13], supplemented with the quantum corrected model (QCM) [14] to account for quantum tunneling. We find that the red-shift of the bonding mode saturates with decreasing distance and an additional tunnel conductivity in the gap region leaves the spectral position unaffected. The TCP modes shift with decreasing gap distance to the red, and the tunnel conductivity damps these modes. All these findings are in perfect agreement with the observations of Esteban et al. [6] for flat gap terminations and would qualify our work as a systematic research paper, if it was not for this single point: despite serious efforts we were unable to confirm the emergence of the low-energy CTP observed by Tan et al. [7] and could not reproduce their simulation results. We will argue why we believe that our results are valid within the electrodynamic and QCM model under consideration, and why a re-interpretation of the experiments might be needed.In our simulations we model the cubes with rounded edges and corners as superellipsoides, whose boundaries are parameterized through u ∈ [0, π) and v ∈ [−π, π) according towhere a determines the cube size (we use side lengths of 35 nm throughout), r is a roundi...

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Gap plasmonics of silver nanocube dimers

et al. 2016

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“…The number of molecules in each hot spot contributing to the total Raman signal is estimated to be around 100 in this system [39]. Identical SERS signals are obtained from each individual investigated NPoM [ Fig.…”

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confidence: 99%

Pulsed Molecular Optomechanics in Plasmonic Nanocavities: From Nonlinear Vibrational Instabilities to Bond-Breaking

et al. 2018

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Small numbers of surface-bound molecules are shown to behave as would be expected for optomechanical oscillators placed inside plasmonic nanocavities that support extreme confinement of optical fields. Pulsed Raman scattering reveals superlinear Stokes emission above a threshold, arising from the stimulated vibrational pumping of molecular bonds under pulsed excitation shorter than the phonon decay time, and agreeing with pulsed optomechanical quantum theory. Reaching the parametric instability (equivalent to a phonon laser or "phaser" regime) is, however, hindered by the motion of gold atoms and molecular reconfiguration at phonon occupations approaching unity. We show how this irreversible bond breaking can ultimately limit the exploitation of molecules as quantum-mechanical oscillators, but accesses optically driven chemistry.

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“…99 Similarly, modifications of the gap can also determine the yields and properties of many optoelectronic processes such as in photoemission 100,101 or in nonlinear plasmonics. 102 However, the sensitivity to fine details of the cavity, even reaching down to atomic-scale variations within the gap, is not always detrimental. If controlled, as for example in a particleon-mirror geometry, the dependence on the faceting can serve as an optical monitor of complex photochemical processes 85 or to trace transport properties at optical frequencies, 29,103 thus allowing access to information that cannot be reached by other techniques.…”

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The Morphology of Narrow Gaps Modifies the Plasmonic Response

et al. 2015

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ABSTRACT:The optical response of a plasmonic gapantenna is mainly determined by the Coulomb interaction of the two constituent arms of the antenna. Using rigorous calculations supported by simple analytical models, we observe how the morphology of a nanometric gap separating two metallic rods dramatically modifies the plasmonic response. In the case of rounded terminations at the gap, a conventional set of bonding modes is found that red-shifts strongly with decreasing separation. However, in the case of flat surfaces, a distinctly different situation is found with the appearance of two sets of modes: (i) strongly radiating longitudinal antenna plasmons (LAPs), which exhibit a red-shift that saturates for very narrow gaps, and (ii) transverse cavity plasmons (TCPs) confined to the gap, which are weakly radiative and strongly dependent on the separation distance between the two arms. The two sets of modes can be independently tuned, providing detailed control of both the near-and far-field response of the antenna. We illustrate these properties also with an application to larger infrared gap-antennas made of polar materials such as SiC. Finally we use the quantum corrected model (QCM) to show that the morphology of the gap has a dramatic influence on the plasmonic response also for subnanometer gaps. This effect can be crucial for the correct interpretation of charge transfer processes in metallic cavities where quantum effects such as electron tunneling are important. KEYWORDS: optical antennas, plasmonic gaps, cavity modes, antenna modes, quantum effects, quantum corrected model, plasmonic resonances, phononic resonances M etallic particles can show a strong optical response due to the excitation of resonant plasmonic modes, making light manipulation possible in structures of dimensions comparable to or smaller than the wavelength. Typical effects are the efficient emission of radiation and the concentration of the incoming light energy in small volumes, thus justifying the characterization of these metallic structures as optical antennas. 1,2 Modifications of the geometry, size, or materials 3−6 affect the optical response. In particular, the resonant modes of optical antennas consisting of two metallic particles separated by a narrow gap show significant sensitivity to the properties of the gap. 7−19 In an optical antenna, a change in geometry can simultaneously affect both the strength and wavelength of its resonances and modify both the far-and near-field response. It is usually a challenge, for example, to control the near field without affecting the far-field properties. In this theoretical work, we discuss how the coexistence 20,21 of two different and mostly spectrally decoupled types of modes in flat-gap antennas, namely, transverse cavity modes 22 and longitudinal antenna modes, 15 allows for a more flexible tuning of far-and near-field properties compared to that of the conventional spherical-gap terminations 13,17,23 The importance of the gap morphology 24 is particularly pronounced for nanometer-...

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Nanooptics of Molecular-Shunted Plasmonic Nanojunctions

Cited by 162 publications

References 43 publications

Gap plasmonics of silver nanocube dimers

Gap plasmonics of silver nanocube dimers

Pulsed Molecular Optomechanics in Plasmonic Nanocavities: From Nonlinear Vibrational Instabilities to Bond-Breaking

The Morphology of Narrow Gaps Modifies the Plasmonic Response

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