Unveiling Plasmonic Dynamics in Hierarchical Gold Superparticles

Plasmonic nanoparticles are attracting much interest in the recent literature [1-3]. Their optical properties have been intensively studied [4-5] by different experimental and theoretical techniques. Recently, superparticles obtained by the hierarchical assemblies of colloidal nanoparticles have shown great promise in transitioning materials from the nanoscale to the mesoscale, building artificial materials with new properties stemming from the crosstalk between constituent nanoparticles. [6] While significant advances have been made in the self-assembly of semiconductor nanoparticles [6-7] the fundamental photophysics which governs their optical response remains largely unclear. In particular, little information is available on the dynamics of photoexcited superparticles made from the ordered assembly of plasmonic nanoparticles.<br/> <br/>We synthesized superparticles (SPs) with a size of about 200-300 nm via by the self-assembly of different size gold nanoparticles (AuNps) by an oil-in-water emulsion template technique. After the synthesis of the SPs the length of the ligands between the nanoparticles has been changed by a ligand exchange protocol, in order to control the distance between individual AuNPs within the SPs, as a handle to control the properties of the final product. The plasmonic properties of the resulting SPs have been studied by femtosecond transient absorption spectroscopy and microscopy in order to reveal the difference between the isolated plasmonic nanoparticles and the SPs and the effects of different assembly conditions.<br/> <br/>The absorption profile of SPs results broader than a colloidal suspension of isolated nanoparticles suggesting that their optical properties are regulated by the interactions between the single constituents rather than a simple sum of isolated AuNPs contributions. Moreover, femtosecond transient absorption (FTA) measurements reveal interesting differences between the relaxation dynamics of bare gold nanocrystals and that to the gold superparticles. In particular, both the spectral position and kinetics of the negative peak of the FTA signal of the superparticles depends on the mutual distance between the nanoparticles. In particular, the timescale typically associated to the electron-phonon scattering decreases as a function of length of the ligands. The results provide new insight on the fundamental electronic dynamics of plasmonic superparticles and pave the way to their applications in nanotechnology.<br/> <br/>[1] Chem. Rev. 2020, 120, 2, 464–525.<br/>[2] Nanoscale Adv., 2020, 2, 3764-3787.<br/>[3] Nat. Comm, 11, 2771, 2020.<br/>[4] Adv. Funct. Mater. 2020, 30, 2005400.<br/>[5] Nanoscale, 2019, 11, 17444-17459.<br/>[6] ACS Nano 2020, 14, 10, 13806–13815.<br/>[7] Nat Synth, 2023, 10.1038/s44160-023-00407-2.