Tunable Plasmon Resonance of Gold Nanoparticles Functionalized by Electroactive Bisthienylbenzene Oligomers or Polythiophene

Abstract

We investigate the effect of new redox molecular switches based on oligothiophene deposited on gold nanoparticles (AuNPs) as thin electroactive layers in the 5−80 nm thickness range. In doing so, we compare systems based on physisorbed electroactive layers (weak electronic coupling) with those based on covalently bonded layers (strong electronic coupling), and we investigate orientation and thickness effects. Two different deposition methods were used. The first is based on bithiophene electropolymerization and the second on diazonium salt electroreduction. In both cases, redox switching of the electroactive layer makes is possible to tune the plasmonic properties of the AuNPs, and the layer thickness has a strong impact on the amplitude of the localized surface plasmon resonance (LSPR) modulation. LSPR modulation upon redox switching also depends on the electronic coupling regime between the AuNP and the organic layer. Indeed, the apparent real part of the dielectric constant seen by the AuNP is larger when oligothiophenes are covalently bonded to the AuNPs. Moreover, the LSPR wavelength, in the 700−750 nm range, shifts in the opposite direction upon redox switching of the organic layers in weak or in strong electronic interaction with the AuNPs. These behaviors may be attributed to orientation effects, but also suggest that, in a strong electronic coupling regime, plasmon delocalization within the covalently grafted conducting organic material is enhanced. ■ INTRODUCTIONA large variety of nanometer-scale devices have been investigated in recent years because of the continuously increasing demand for ultimate miniaturization of electronic and photonic systems. Among these, devices based on gold nanoparticles (AuNPs) are well-known for their remarkable properties. Indeed, AuNPs smaller than the incident light wavelength exhibit coherent oscillations of the confined free electrons in their conduction bands. When the frequency of these collective electron oscillations coincides with that of the excitation light, a resonance phenomenon appears and strong absorption in the visible range occurs. The frequency of this socalled localized surface plasmon resonance (LSPR) depends on the size of the NPs, their shape, the distance between them, and the dielectric constant of the surrounding medium. Such LSPR enhances electric fields very close to the NP structures and allows the manipulation of light and its interaction with matter at the nanoscale. In this sense NPs work in a similar way to that of antennas in radio and telecommunication systems, but at optical frequencies, i.e., at frequencies corresponding to typical electronic excitations in matter. Such NP-based systems are part of the emerging scientific domain of plasmonics which offer an opportunity to merge photonics and electronics at nanoscale dimensions to obtain unusual properties for unprecedented levels of synergy between optical and electronic functions. Plasmonic devices such as waveguides, 1−3 filters, 4,5 polarizers, 4,6 light sources, 7 lenses, 8,9 and a...

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