Electric Behavior of Noble Metal Nanoparticles Based Networks Stabilized by Bifunctional Thiols
Sara Cerra a, Raoul Fioravanti a, Grigorian Souren b, Loreta Angela Muscarella c, Ilaria Fratoddi a
a Dept. of Chemistry, University of Rome Sapienza, P.le A. Moro 5, 00185 Rome,Italy
b Dept. of Physics, University of Siegen, Walter-Flex-Straße 3, D-57072 Siegen,Germany
c Center for Nanophotonics, AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Sara Cerra, 261
Publication date: 11th February 2019

Functionalized Metal Nanoparticles (MNPs) have recently become established as novel hybrid materials for optoelectronics applications, thanks to a large 3D surface area that leads to high local concentrations of ligands on the surface [1]. A fundamental role is played by functionalizing ligand and the nanoparticle networks can be utilized as a template  to incorporate single molecule and studied as photoconducting materials, in which the nanoparticles act as electronic contacts to the molecule [2].

In this work, the optical features of nanometer sized metal particles have been employed to design hybrid conductive systems capable of enhancing the optical/electronic properties of the metallic core tuned together with the active optical behavior of the ligands. Gold and silver nanoparticles (AuNPs and AgNPs) stabilized by a fluorene thiolate derivative 9,9-didodecyl-2,7-bis(acetylthio)fluorene (FL) were prepared. The hybrid compounds form a regular networks of MNPs separated each other by FL spacers covalently linked by thiol groups.

The colloidal synthesis of Au/Ag networks was implemented through a Brust-Shiffrin procedure. Briefly, metal cation (Au3+ or Ag+) is phase transferred into toluene from water using tetraoctylammonium bromide (TOAB) as phase transfer agent. This is followed by the addition of 9,9-didodecyl-2,7-bis(acetylthio)fluorene dissolved in toluene under inert atmosphere and NaBH4 as reducing agent. In the synthesis process MNPs size results to be around 5-10 nm in diameter as measured by UV-Vis, DLS, TEM and FESEM.                                                                                 

A versatile hybrid optoelectronic devices based on AuNPs and AgNPs stabilized by bifunctional organic ligand was obtained. Functionalized MNPs deposited on interdigitated electrodes by drop casting as a thin film, at room temperature shows a Current-Voltage (I/V) response curve tunable as a function of metallic core of the nanoparticles. The electrical properties of these systems have been investigated by conductivity measurement in dark/under illumination condition , pointing out the response at different environments. Preliminary measurements on AuNPs-FL shows that the I/V characteristic is completely symmetrical with respect to the polarity of the applied voltage, typical behaviour of semiconductor materials [3]. The curve follows non-ohmic relation which depends on the conjugated system of Fluorene bridge, but no differences were observed in the dark/under illumination conditions. For a detailed investigation of the structural changes occurring in the AuNPs network was studied with GIXD. The line profiles were analyzed in out-of-plane and in-plane directions and data support the formation of a uniform network that after annealing at 100°C were extended from hexagonal to cubic arrangement, without decomposition [4].

In order to further study the conductivity of the MNPs-FL material, conductivity measurements of AgNPs-FL were performed. The electrical behaviour of the AgNPs-FL sample shows the ability to respond to different illumination conditions. Specifically, the conductivity is lower in the dark, probably due to an oxidation process, but increases under light illumination. Consequently, the I/V curve lost symmetry driven by rectification effect typical of diodes.

In conclusion, flexible electronic devices with high level of miniaturization has been designed. The electrical properties of the systems were tested at room temperature with respect to different illumination conditions. Moving towards optoelectronics, our nano-electronics devices behaves as prototype circuit in which conducibility value are depends on the metallic core and the thickness of the layer deposited on the interdigitated electrode surface. Although the AuNPs-FL network does not show significant differences in the dark/light semiconductor response, selective oxidation of AgNPs network gives rise to a diode behaviour in the I-V characteristic that could find potential applications in optoelectronics.

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