Article pubs.acs.org/JPCC
Tunable Plasmon Resonance of Gold Nanoparticles Functionalized by Electroactive Bisthienylbenzene Oligomers or Polythiophene Delphine Schaming, Van-Quynh Nguyen, Pascal Martin, and Jean-Christophe Lacroix* Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, Université Paris Diderot, 15 rue Jean-Antoine de Baïf, 75205 Paris, Cedex 13, France S Supporting Information *
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.
■
fields, the large sensitivity of the LSPR with NPs environment has also allowed their use as chemical12,13 and biological14,15 sensors. It has also recently been used to sense the high-to-low spin transition of a thin film of a transition metal complex.16 In order to become a highly valuable technology, plasmonic switches are needed and have been developed. In such systems, an external stimulus can reversibly tune the frequency and/or the amplitude of the LSPR. While strategies based on tuning the size or the shape of NPs appear not easy for a reversible control of the LSPR, a new class of active molecular plasmonic devices based on metallic NPs surrounded by switchable systems shows particular interest. Indeed, in such systems, the switch can tune the LSPR of the NPs by changing the effective dielectric constant of their surrounding medium. Several external stimuli have already been employed, based on thermo-,17 pH-,18 photo-,19 phase change-,20,21 magnetic-,22 electrical-,23 and redox-responsive layers or molecules. In particular, electrochemical switching, using redox-sensitive layers, appears as a useful tool to reversibly control the properties of metallic NPs, leading to electrochemically driven active plasmonic devices. Wang and Chumanov were the first to monitor the surface plasmon resonance of an AgNP array coated by an electroactive layer, namely a thin film of tungsten oxide WO3 with a thickness of ca. 120 nm.24 Reduced and
INTRODUCTION A 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 antennas10,11 have been reported. Apart from information processing and optical © 2014 American Chemical Society
Received: July 18, 2014 Revised: October 3, 2014 Published: October 6, 2014 25158
dx.doi.org/10.1021/jp507210t | J. Phys. Chem. C 2014, 118, 25158−25166
The Journal of Physical Chemistry C
Article
Scheme 1. Representation of the Two Systems under Investigation, Their Organization on the Surface, and Their Reduced (Nonconducting) State and Oxidized (Conducting) State: (Left) Physisorbed Polythiophene; (Right) Grafted Oligo(BTB)
conductance switch between conducting and totally blocking states depending on the voltage. Thicknesses can easily be varied from 5 to 20 nm, and the grafted oligo(BTB)34 and the metal are in the strong electronic coupling regime, thanks to the covalent grafting of the thiophene-based oligomers on the metallic surface. Such ultrathin layers, in which the πconjugated molecules are, to some extent, oriented, can be integrated into molecular electronic devices,35 exhibit unusual transport properties in metal/molecular/metal junctions,36 and their electroactive behavior is similar to that of physisorbed conducting polymers based on polythiophene. We will therefore investigate how the LSPR of AuNP arrays (obtained by electron beam lithography (EBL)) can be modulated by an ultrathin electrochemically tunable layer of oligo(BTB) of various thicknesses. The results will be compared with those obtained with physisorbed conducting polymers based on polythiophene directly electropolymerized on the plasmonic electrode. Scheme 1 summarizes the two systems under investigation, their organization on the surface, and the structural differences between their oxidized and reduced forms.
oxidized forms of WO3 having different electrical conductivities and thus different dielectric constants, the LSPR of the NPs was reversibly modulated on a 30 nm range by switching the externally applied potential. Following that, we have reported the use of thin layers of electroactive conducting polymers, such as polyaniline (PANI)25−28 or poly(3,4-ethylenedioxythiophene) (PEDOT),29 deposited on AuNP gratings. Switching from the reduced nonconducting state to the oxidized conducting state of PANI led also to a change in the effective dielectric constant, and blue-shifts of 17 and 81 nm of the LSPR bands were observed using polymer films with thicknesses of ca. 50 and 100 nm, respectively.27 In the case of PEDOT, a giant blue-shift of 192 nm was even measured for a film thickness of about 150 nm.29 Similar active plasmonic devices were obtained by Baba et al.30 In such studies, the electroactive layers were weakly bonded to the NP, and the interfaces between the NP and the electroactive layers were not controlled. When the film thickness is decreased, the effect of the molecular switch on the LSPR is generally much smaller. Zheng and co-workers have reported the sole example of electroactive molecular plasmonic device relying on AuNPs coated with a thin monolayer of molecules (
