Gap plasmonics of silver nanocube dimers

Abstract

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...

Quantum Plasmonics

“…This intriguing combination of molecular electronics with plasmonics was shown by Nijhuis' group who observed, using EELS, a charge transfer plasmon up to a separation of 1.3 nm for silver nanocubes functionalised with a monolayer of self-assembled molecules (SAMs) [139]. They found that the frequency of the CTP was dependent on the type of bridging molecule, for instance the size of the molecules HOMO-LUMO gap will dictate the tunnelling rate (we take this oportunity to point out that there has been some dispute of their intepretation of the experimental results regarding the CTP using the quantum corrected model [70]). This opens up the possibility of new biosensors that use changes in the tunnelling current to detect chemical concentrations and processes, for instance Benz et al have shown that difference of a thiol group (which allows the formation of a covalent bond to gold and hence can allow charge transfer) between biphenyl-4-thiol and biphenyl-4,4'-dithiol results in a shift of the coupled plasmon mode depending on the mole fraction of the two molecules [11].…”

Localized Plasmonic Resonances of Prolate Nanoparticles in a Symmetric Environment: Experimental Verification of the Accuracy of Numerical and Analytical Models

“…The MNPBEM code developed by Hohenester and Trügler [89][90][91], which in recent years has been extensively used for the simulation of plasmonic nanoparticles [92][93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108], is used for these simulations. Further details can be found in Sec.…”

Ab initioapproach for gap plasmonics

“…As mentioned before, the gap conductivity mainly acts as a trigger for the appearence of the CTP peak [13,41], however, the precise σ t value is of only minor importance. Other quantities, such as the amount of charge transferred across the gap, could be influenced more strongly by σ t .…”

“…This is finding is in accordance to Refs. [13,41] where it was shown that σ t mainly triggeres the emergence of the CTP: below a critical value the CTP peak is absent, above the critical value the peak appears but its spectral position and shape is not substantially influenced by the precise magnitude of σ t .…”

“…For gap distances in the sub-nanometer range electrons can tunnel through the gap region, leading to the emergence of new charge transfer plasmons (CTPs) [3][4][5] which have been observed optically [6,7] and in electron energy loss spectroscopy (EELS) [8,9]. Tunneling through larger gap regions has been demonstrated in molecular tunnel junctions [10][11][12][13].…”