Photochemistry and phOtophysics of Plasmons towards fully COntRolled Nanolocalized polymerization – POPCORN

Optically excited plasmonic nanoparticles (NPs) can advantageously be used to activate chemical transformations directly and locally on their surfaces, opening interesting prospects in many areas. Despite numerous demonstrations, the exact mechanisms and main parameters driving the control of plasmon-mediated photochemical reactions remain however unclear and controversial. In addition, the efficiency of such reactions are most often difficult to rationalize. The aim of POPCORN is to get a full understanding of plasmon-driven polymerization in a well-controlled environment. To achieve this objective a consortium gathering experts in both photophysics and photochemistry has been set up: Partner 1 (P1, CEA-SPEC, Nanophotonics & plasmonics), who will coordinate this project; Partner 2 (P2, IS2M, photochemistry); Partner 3 (P3, CEA-NIMBE, colloidal NP synthesis) and Partner 4 (P4, UTT-ICD-L2n, Surface Enhanced Raman Spectroscopy (SERS)). All partners are used to work together.
The objective of POPCORN is to investigate and quantify the role and potential synergies of the three main processes -photon, hot charge carrier and heat-driven effects- taking place during plasmon-mediated polymerization (PMP) at the surface of a plasmonic NP excited at resonance. For this purpose, we propose systematic studies down to the single object level, considering specifically selected model gold crystalline NPs that will be synthesized using colloidal chemistry (P3), in association with specifically designed polymerisable formulations exhibiting differential sensitivities to the photon (i), charge (ii) and thermally (iii) triggered processes (P2). Both the yield and the nanoscale localization of the polymer product resulting from PMP will be determined using high resolution transmission (TEM) electron microscopy (P2). Before such “post-mortem” topography characterization, the PMP around model NPs will be monitored in-situ in real time using dark-field and luminescence spectroscopy (P1). A specific two-color SERS setup will also be developed in order to follow the chemical changes of the molecular components surrounding the NPs during PMP.
POPCORN will moreover benefit from fundamental characterization inputs of the NP plasmonic relaxation physics. Distribution and dynamics of hot carrier emission in bare NPs will be determined by time-resolved photoelectron emission microscopy (PEEM, P1). Local temperature at NP surrounding will be accessed through SERS measure of the Stokes and anti-Stokes emission band ratio (P4).
For each reference system (model NP / polymerisable formulation), the above mentioned experiments will be systematically performed enabling the rationalized study of different degrees of freedom : from the influence of the excitation conditions (wavelength, irradiance, continuous wave/pulsed excitation …) to the influence of the NP interface (presence of surfactant or insulating silica layers, the latter preventing any charge transfer processes).
Beyond the quantification of the relative contribution of each process (i-iii), special attention will also be paid to the characterization of their potential interplay. The outcome will be the determination of the key parameters enabling to optimize the reactivity and fully control nanoscale polymerization around NPs exploiting plasmon relaxation. Although of a rather fundamental nature, POPCORN will lead to precisely defined rules leading to perfectly controlled and optimized processes, which will not only enrich the toolbox of nanofabrication but should also more broadly enable new developments in photocatalysis.