Ultrafast versus Continuous-Wave Plasmonics: How Heat and Hot Electrons Drive Polymerization

Hot carrier and heat generation in plasmonic nanoparticles are critically important in plasmon-assisted chemistry and catalysis, sensing, and optoelectronic applications. The selective control of these effects plays a key role in determining the efficiency of chemical processes and the performance of associated devices. A major challenge lies in distinguishing and precisely controlling these effects under experimental conditions, particularly since most applications rely on continuous-wave (CW) irradiation, where both phenomena occur simultaneously. In this work, we investigate the polymerization of diazonium salts on gold nanocubes under CW and femtosecond (fs) irradiation as a probe for evaluating the efficiency of hot-electron generation and heat production. Our findings reveal that continuous excitation of the quadrupole plasmonic mode promotes a hot-electron-driven process, owing to the absorptive character of the quadrupole resonance. Conversely, excitation of the dipole mode under CW irradiation results in chaotic polymer growth, driven by lattice heat generation. However, fs excitation of the dipole mode induces polarization-dependent polymerization precisely at plasmonic hot spots. We attribute this phenomenon to the synergistic effect of vibrational heating of adsorbate molecules occurring on the picosecond scale due to energy transfer from hot electrons, which leads to the formation of the first polymer layers on the nanocube corners, followed by a lattice-heating-driven reaction that proceeds more rapidly at the created anchoring sites. These results offer insights into the ultrafast processes occurring in gold nanoparticles upon irradiation and pave the way for designing energetically efficient chemical reactions and optoelectronic devices.